Filed Pursuant to Rule 424(b)(3)

Registration No. 333-185376

Prospectus Supplement No. 3

(To Prospectus dated June 4, 2013)

Dynegy Inc.

32,459,817 Shares Common Stock

This Prospectus Supplement, dated August 15, 2013 (the “Supplement”), is being filed to update, amend and supplement the information previously included in our prospectus, dated June 4, 2013 (the “Prospectus”), with the financial information included herein.  This Supplement is not complete without, and may not be delivered or used except in connection with, the Prospectus, including all amendments and supplements thereto.  The Prospectus relates to the resale of up to 32,459,817 shares of our common stock by the selling stockholder named in the Prospectus.

Our common stock is currently listed on the New York Stock Exchange, which we refer to as the NYSE, under the symbol “DYN.”  On August 14, 2013, the last reported sale price on the NYSE of our common stock was $19.71.

Investing in our common stock involves risks.  See “Risk Factors” beginning on page 9 of the Prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete.  Any representation to the contrary is a criminal offense.

This Prospectus does not constitute an offer to sell or the solicitation of an offer to buy any securities.

The date of this Supplement is August 15, 2013.

Table of Contents

Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

Table of Contents

Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Index

Three and Six Months Ended June 30, 2013, and 2012

Table of Contents

GLOSSARY OF TERMS AND ABBREVIATIONS

2012 Annual Report - The Ameren Energy Resources Company, LLC audited consolidated financial statements and accompanying notes for the year ended December 31, 2012.

Ameren - Ameren Corporation and its subsidiaries on a consolidated basis. In references to financing activities, acquisition activities, or liquidity arrangements, Ameren is defined as Ameren Corporation, the parent.

Ameren Illinois - Ameren Illinois Company, an Ameren Corporation subsidiary that operates a rate-regulated electric and natural gas transmission and distribution business in Illinois, doing business as Ameren Illinois.

Ameren Missouri - Union Electric Company, an Ameren Corporation subsidiary that operates a rate-regulated electric generation, transmission and distribution business, and a rate-regulated natural gas transmission and distribution business in Missouri, doing business as Ameren Missouri.

AER — Ameren Energy Resources Company, LLC, an Ameren Corporation subsidiary that operates a merchant electric generation and marketing business in Illinois, including Genco, AERG, and Marketing Company. Medina Valley was a subsidiary of AER through March 13, 2013. It was distributed from AER to Ameren on March 14, 2013.

AERG - AmerenEnergy Resources Generating Company, an AER subsidiary, that operates a merchant electric generation business in Illinois.

ARO - Asset retirement obligations.

AMS - Ameren Services Company, an Ameren Corporation subsidiary that provides support services to Ameren and its subsidiaries.

CAIR - Clean Air Interstate Rule.

CCR - Coal combustion residuals.

CILCO - Central Illinois Light Company, a former Ameren Corporation subsidiary that operated a rate-regulated electric transmission and distribution business, a merchant electric generation business through AERG, and a rate-regulated natural gas transmission and distribution business. CILCO merged with, and into CIPS, which was renamed Ameren Illinois, on October 1, 2010. Immediately after the merger, Ameren Illinois distributed the common stock of AERG to Ameren Corporation.

CIPS - Central Illinois Public Service Company, an Ameren Corporation subsidiary, which was renamed Ameren Illinois Company in 2010. CIPS’ energy centers were transferred to Genco in 2000.

CO2 - Carbon dioxide

CSAPR - Cross-State Air Pollution Rule.

Dynegy — Dynegy Inc.

EEI - Electric Energy, Inc., an 80%-owned Genco subsidiary that operates merchant electric generation energy centers and FERC-regulated transmission facilities in Illinois. The remaining 20% ownership interest is owned by Kentucky Utilities Company, a nonaffiliated entity.

EPA - Environmental Protection Agency, a United States government agency.

FASB - Financial Accounting Standards Board, a rulemaking organization that establishes financial accounting and reporting standards in the United States.

FERC - Federal Energy Regulatory Commission, a United States government agency.

GAAP - Generally accepted accounting principles in the United States of America.

Genco - Ameren Energy Generating Company, an AER subsidiary that operates a merchant electric generation business in Illinois and holds an 80% ownership interest in EEI.

IPH - Illinois Power Holdings, LLC, an indirect wholly owned subsidiary of Dynegy Inc.

Marketing Company - Ameren Energy Marketing Company, an AER subsidiary that markets power for Genco, AERG, and EEI.

MATS - Mercury and Air Toxics Standards.

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Medina Valley - AmerenEnergy Medina Valley Cogen LLC, an AER subsidiary through March 13, 2013, which owned a 40-megawatt natural gas-fired electric energy center. This energy center was sold in February 2012. This company was distributed from AER to Ameren on March 14, 2013.

Megawatthour or MWh - One thousand kilowatthours.

Moody’s - Moody’s Investors Service Inc., a credit rating agency.

MPS - Multi-Pollutant Standard, a compliance alternative within Illinois law covering reductions in emissions of SO2, NOx, and mercury, which Genco, EEI, and AERG elected in 2006.

New AER - A limited liability company to be formed as a direct wholly owned subsidiary of AER. New AER will be acquired by IPH and will include substantially all of the assets and liabilities of AER, except for certain assets and liabilities retained by Ameren.

NOx -  Nitrogen oxide.

NPNS - Normal purchases and normal sales.

NSR - New Source Review provisions of the Clean Air Act, which include Nonattainment New Source Review and Prevention of Significant Deterioration regulations.

OCI - Other comprehensive income (loss) as defined by GAAP.

PUHCA 2005 - The Public Utility Holding Company Act of 2005.

S&P - Standard & Poor’s Ratings Services, a credit rating agency.

SO2 - Sulfur dioxide.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Consolidated Statement of Operations and Comprehensive Income (Loss)

(Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

The accompanying notes are an integral part of these consolidated financial statements

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Consolidated Balance Sheet (Unaudited)

June 30, 2013, and December 31, 2012

The accompanying notes are an integral part of these consolidated financial statements

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Consolidated Statement of Cash Flows (Unaudited)

Six Months Ended June 30, 2013, and 2012

The accompanying notes are an integral part of these consolidated financial statements.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

1.                            Summary of Significant Accounting Policies

General

AER is a subsidiary of Ameren, a public utility holding company under PUHCA 2005, administered by the FERC. AER is a merchant electric generation and marketing business operating in Illinois. AER is headquartered in Collinsville, Illinois. AER’s principal subsidiaries are Genco, AERG, Medina Valley (through March 13, 2013), and Marketing Company. Genco has an 80% ownership interest in EEI. AER consolidates EEI for financial reporting purposes. These consolidated financial statements present the consolidated results of operations, financial position and cash flows of AER.

In December 2012, Ameren determined that it intended to, and it was probable that it would, exit its merchant generation business, of which AER is a part. Based on the expectation of reduced financial support from Ameren, together with existing power market conditions and cash flow requirements, AER estimated, at that time, that it was more likely than not that Genco would sell its Elgin energy center for liquidity purposes within two years. This change in assumption resulted in a pretax noncash long-lived asset impairment of $70 million being recorded in the fourth quarter of 2012 relating to the Elgin energy center. AER’s long-lived assets were not classified as held for sale as of December 31, 2012, under authoritative accounting guidance as all criteria to qualify for that presentation were not met as of December 31, 2012. Specifically, AER did not consider it probable at December 31, 2012, that a disposition of an energy center would occur within one year.

On March 14, 2013, Ameren entered into a transaction agreement to divest its merchant generation business through New AER to IPH. Immediately prior to Ameren’s entry into the transaction agreement with IPH, on March 14, 2013, Genco exercised its option under the amended put option agreement with Medina Valley and received an initial payment of $100 million for the pending sale of Genco’s Elgin, Gibson City, and Grand Tower gas-fired energy centers to Medina Valley, which is subject to FERC approval. As a result, AER determined that the assets and liabilities associated with the Elgin, Gibson City, and Grand Tower gas-fired energy centers qualified for held for sale presentation as of March 31, 2013, and continued to meet the held for sale criteria as of June 30, 2013. Therefore, the assets and liabilities associated with these gas-fired energy centers were segregated and presented separately as held for sale as of June 30, 2013, and comparatively at December 31, 2012. The operating results of the Elgin, Gibson City, and Grand Tower gas-fired energy centers did not qualify for discontinued operations presentation because AER will continue to sell power into the same markets with its remaining generation assets. See Note 2 - Assets Held for Sale for additional information regarding that presentation.

AER’s accounting policies conform to GAAP. The financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in management’s opinion, for a fair presentation of AER’s results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the 2012 Annual Report.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

Basis of Presentation

These consolidated financial statements reflect the historical results of operations, financial position, and cash flows of AER for the periods presented. The consolidated statements of operations and comprehensive income (loss) reflect intercompany expense allocations made to AER by AMS, an Ameren subsidiary that provides support services to Ameren and its subsidiaries, and by other Ameren affiliated entities for certain corporate functions historically provided by these entities during the periods presented. While management considers these allocations to have been made on a reasonable basis, the allocations presented do not necessarily reflect the expenses that would have been incurred had AER operated as a stand-alone business. See Note 7 — Related Party Transactions for further information on expenses allocated to AER. Interest expense shown in the consolidated statements of operations reflects the interest expense associated with the aggregate direct third-party borrowings and interest-bearing amounts due to affiliate borrowings for each period presented. Additionally, the consolidated financial statements include the costs associated with AER’s participation in Ameren’s single-employer pension and postretirement benefit plans. The consolidated financial position, results of operations and cash flows of AER could differ from those that would have resulted had AER operated autonomously or independently of Ameren and its subsidiaries.

See Glossary of Terms and Abbreviations at the beginning of this report for a definition of terms and abbreviations used throughout this report.

Uncertain Tax Positions

The amount of unrecognized tax benefits as of June 30, 2013, was $12 million. The amount of unrecognized tax benefits as of June 30, 2013, that would impact the effective tax rate, if recognized, was $1 million.

AER is included in Ameren’s federal income tax return. Ameren’s federal income tax returns for the years 2007 through 2011 are before the Appeals Office of the Internal Revenue Service. Ameren’s federal income tax return for the year 2012 is currently under examination.

It is reasonably possible that a settlement will be reached with the Appeals Office of the Internal Revenue Service in the next 12 months for the years 2007 through 2010. This settlement, primarily related to uncertain tax positions for capitalization versus currently deductible repair expense and research tax deductions, is expected to result in a decrease in uncertain tax benefits of approximately $12 million. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of unrecognized tax benefits to increase or decrease. However, AER does not believe any such increases or decreases, including the decrease from the reasonably possible IRS Appeals Office settlement discussed above, would be material to AER’s results of operations, financial position, or liquidity.

State income tax returns are generally subject to examination for a period of three years after filing of the return. AER does not currently have material state income tax issues under examination, administrative appeals, or litigation. The state impact of any federal changes remains subject to examination by various states for a period of up to one year after formal notification to the states.

Noncontrolling Interest

AER’s noncontrolling interest comprised the 20% of EEI’s net assets not owned by AER. This noncontrolling interest is classified as a component of equity separate from AER’s equity in its consolidated balance sheet. A reconciliation of the equity changes attributable to the noncontrolling interest at AER for the three and six months ended June 30, 2013, and 2012, is shown below:

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

Asset Retirement Obligations

The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the six months ended June 30, 2013:

(a)              Balance included $7 million in “Other Current Liabilities” on the balance sheet at December 31, 2012. This balance was reclassified as a long-term liability as of June 30, 2013.

(b)              Less than $1 million.

(c)               AER changed its estimate related to updated retirement dates for certain CCR storage facilities during the first quarter of 2013. See Note 8 — Commitments and Contingencies for additional information.

On March 14, 2013, Ameren entered into a transaction agreement to divest its merchant generation business through New AER to IPH. Under the terms of that agreement, Ameren will retain the existing AROs associated with the Meredosia and Hutsonville energy centers, which were estimated at $27 million as of June 30, 2013. AROs associated with the Meredosia and Hutsonville energy centers will continue to be included in “Asset retirement obligations” on AER’s consolidated balance sheet until they are transferred to Ameren prior to the transaction agreement closing. The ARO associated with the Grand Tower energy center of $10 million was included in “Current liabilities held for sale” as of June 30, 2013, and December 31, 2012. See Note 2 - Assets Held for Sale for information regarding Genco’s sale of the Grand Tower energy center to Medina Valley and the transfer of the Meredosia and Hutsonville energy centers to Medina Valley.

Retirement Benefits

AER employees participate in various pension and postretirement plans. These include plans exclusively for AER employees as well as those in which AER employees participate along with other Ameren employees. The primary objective of these plans is to provide eligible employees with pension and postretirement health care and life insurance benefits. The cost of these plans is included or allocated in the accompanying statement of operations and comprehensive income (loss).

On March 14, 2013, Ameren entered into a transaction agreement to divest its merchant generation business through New AER to IPH. Under the terms of that agreement, Ameren will retain the portion of AER’s pension and postretirement benefit obligations associated with current and former employees that are included in the Ameren Retirement Plan, the Ameren Supplemental Retirement Plan, the Ameren Retiree Medical Plan, and the Ameren Group Life Insurance Plan. New AER will retain the pension and other postretirement benefit obligations associated with EEI’s current and former employees that are included in the Revised Retirement Plan for Employees of Electric Energy, Inc., the Group Insurance Plan for Management Employees of Electric Energy, Inc., and the Group Insurance Plan for Bargaining Unit Employees of Electric Energy, Inc. These EEI obligations were estimated at $37 million at June 30, 2013, and were included in “Pension and other postretirement benefits” on our consolidated balance sheet. New AER will also retain the $15

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

million asset relating to the overfunded status of one of EEI’s postretirement plans, which was included in “Other assets” on AER’s consolidated balance sheet at June 30, 2013.

As discussed above, employees of AER also participate in Ameren’s defined benefit pension and postretirement plans that cover other non-AER Ameren employees. The cost of these benefits allocated to AER also includes the pension and postretirement costs of AMS employees working on behalf of AER. These costs are included in the consolidated statement of operations and comprehensive income (loss) within “Other operations and maintenance” expenses. The following table presents the cost of these benefits during the three and six months ended June 30, 2013 and 2012:

(a)         Less than $1 million

The “Accrued pension and other postretirement benefits” liability as of June 30, 2013, and December 31, 2012, included in the accompanying consolidated balance sheet does not include the unfunded liabilities associated with Ameren’s pension and postretirement benefit plans. The “Pension and other postretirement benefits” line only includes the unfunded liabilities associated with the EEI pension and postretirement benefit plans. The balance sheet line item “Accounts payable — affiliates” included $10 million and $12 million associated with the net payable to Ameren Corporation for the obligations of AER employees that participated in Ameren’s single-employer pension and postretirement plans as of June 30, 2013, and December 31, 2012, respectively.

The EEI Plans section below relates only to EEI pension and postretirement benefit plans and does not include information relating to Ameren pension and postretirement benefit plans.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

EEI Plans

The following table presents the components of EEI’s net periodic pension cost for the three and six months ended June 30, 2013, and 2012:

The following table presents the components of the net periodic postretirement benefit cost for the three and six months ended June 30, 2013, and 2012:

Accounting and Reporting Developments

The following is a summary of recently adopted authoritative accounting guidance that could impact AER:

Presentation of Comprehensive Income

In June 2011, FASB amended its guidance on the presentation of comprehensive income in financial statements. The amended guidance changed the presentation of comprehensive income in the financial statements. It requires entities to report components of comprehensive income either in a continuous statement of comprehensive income or in two separate but consecutive statements. This guidance was effective for AER beginning in the first quarter of 2012 with retroactive application required. The implementation of the amended guidance did not affect results of operations, financial position, or liquidity.

In February 2013, the FASB amended this guidance to require an entity to provide information about the amounts reclassified out of accumulated OCI by component. In addition, an entity is required to present significant amounts reclassified out of accumulated OCI by the respective line items of net income either on the face of the statement where net income is presented or in the footnotes. The implementation of this amended guidance in the first quarter of 2013 did not affect AER’s results of operations, financial position, or liquidity, as this guidance only requires additional disclosures and substantially all the information that this amended guidance requires is already disclosed elsewhere in the financial statements. The only significant amounts reclassified out of accumulated OCI were related to gains and losses on cash flow hedges. See Note 5 - Derivative Financial Instruments for the required disclosures.

Disclosures about Offsetting Assets and Liabilities

In December 2011, FASB issued additional authoritative accounting guidance to improve information disclosed about financial and derivative instruments. The guidance requires an entity to

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

disclose information about offsetting and related arrangements to enable users of the financial statements to understand the effect of those arrangements on financial position. In January 2013, FASB amended this guidance to limit the scope to derivative instruments, repurchase agreements and reverse repurchase agreements, and securities borrowing and securities lending transactions.  The additional guidance did not affect AER’s results of operations, financial position, or liquidity, as this guidance only requires additional disclosures. AER adopted this guidance for the first quarter of 2013. See Note 5 — Derivative Financial Instruments for the required disclosures.

Presentation of an Unrecognized Tax Benefit

In July 2013, FASB issued additional authoritative accounting guidance to provide explicit guidance on the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The objective of this guidance is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits. It requires entities to present an unrecognized tax benefit as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward to the extent a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is available under the tax law. The amended guidance will not affect AER’s results of operations, financial position, or liquidity, as this guidance is presentation-related only. This guidance will be effective for AER beginning in the first quarter of 2014.

2.                            Assets Held for Sale

Transaction Agreement with IPH

On March 14, 2013, Ameren entered into a transaction agreement to divest its merchant generation business through New AER to IPH. Under the terms of the transaction agreement, AER will effect a reorganization that will, among other things, transfer substantially all of the assets and liabilities of AER, other than (i) any outstanding debt obligations of AER to Ameren or its other subsidiaries, except for certain intercompany balances discussed below, (ii) all of the issued and outstanding equity interests in Medina Valley, (iii) the assets and liabilities associated with Genco’s Meredosia, Hutsonville, Elgin, Gibson City, and Grand Tower energy centers, (iv) the obligations relating to Ameren’s single-employer pension and postretirement benefit plans, and (v) the deferred tax positions associated with Ameren’s ownership of these retained assets and liabilities, to New AER. IPH will acquire all of the equity interests in New AER. On March 14, 2013, AER transferred its interest in Medina Valley at carrying value to Ameren. The assets of Medina Valley primarily consisted of $13 million in advances to the non-state-regulated subsidiary money pool.

Ameren will retain the portion of AER’s pension and postretirement benefit obligations associated with current and former employees that are included in the Ameren Retirement Plan, the Ameren Supplemental Retirement Plan, the Ameren Retiree Medical Plan, and the Ameren Group Life Insurance Plan. New AER will retain the pension and other postretirement benefit obligations associated with EEI’s current and former employees that are included in the Revised Retirement Plan for Employees of Electric Energy, Inc., the Group Insurance Plan for Management Employees of Electric Energy, Inc., and the Group Insurance Plan for Bargaining Unit Employees of Electric Energy, Inc. These EEI obligations are estimated at $37 million at June 30, 2013. New AER will also retain the estimated $15 million asset at June 30, 2013, relating to the overfunded status of one of EEI’s postretirement plans.

Ameren will retain AER’s Meredosia and Hutsonville energy centers, which are no longer in operation and had an immaterial property and plant asset balance as of June 30, 2013. Ameren will also retain AROs associated with these energy centers, estimated at $27 million as of June 30,

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

2013. All other AROs associated with AER are expected to be assumed by either IPH or the third-party buyer of the Grand Tower energy center. Upon the transaction agreement closing, with the exception of certain agreements, such as supply obligations to Ameren Illinois, a note from New AER to Ameren relating to cash collateral that will remain outstanding at closing, and Genco money pool advances, all intercompany agreements and debt between AER and its subsidiaries, on the one hand, and Ameren and its non-AER affiliates, on the other hand, will be either retained or cancelled by Ameren, without any cost or obligation to IPH or New AER and its subsidiaries. Immediately prior to the transaction agreement closing, the cash collateral provided to New AER by Ameren through money pool borrowings will be converted to a note payable to Ameren, which will be payable, with interest, 24 months after closing or sooner as cash collateral requirements are reduced. Cash collateral postings by AER and its subsidiaries with external parties, including postings related to exchange-traded contracts, at June 30, 2013, were $29 million. Ameren will retain Genco’s tax payable to Ameren Illinois, which was $44 million as of June 30, 2013.

Ameren’s retention of AER’s liabilities for pension and postretirement benefit obligations relating to Ameren’s plans, the Meredosia and Hutsonville energy centers and those two energy centers’ related AROs, the tax payable to Ameren Illinois, and related deferred tax balances associated with each transferred balance will be accounted for as transactions between entities under common control and transferred at their carrying values.

In addition, if Ameren’s divestiture of New AER to IPH is completed, AER expects the tax basis of property, plant and equipment to decrease and deferred tax assets related to federal and state income tax net operating loss carryforwards and income tax credits to decrease with corresponding offsets to equity. The amount of any such decrease is dependent on the value and timing of the New AER divestiture transaction. As of June 30, 2013, AER had approximately $155 million in federal income tax net operating loss carryforwards and $1 million in federal income tax credit carryforwards. These carryforwards are expected to offset income tax liabilities into 2015, consistent with the tax allocation agreement with Ameren. If AER is no longer a subsidiary of Ameren, the tax allocation agreement will terminate and it is probable that some or all of AER’s tax carryforwards will not be utilized.

As described in more detail below under “Amended Put Option Agreement, Asset Purchase Agreement and Guaranty” as a condition to the transaction agreement, Genco will receive cash proceeds from the exercise of its option under the March 28, 2012 put option agreement, as amended, for the sale to Medina Valley of the Elgin, Gibson City, and Grand Tower gas-fired energy centers in an amount equal to the greater of $133 million or the appraised value of such energy centers. If these gas-fired energy centers are subsequently sold by Medina Valley within two years of the put option closing, Medina Valley will pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the amount it paid at the asset purchase agreement closing. Ameren has commenced a sale process for these energy centers and expects a third-party sale will be completed during 2013.

Genco’s $825 million in aggregate principal amount of senior notes will remain outstanding following the closing of the transaction agreement and will continue to be solely obligations of Genco. Pursuant to the transaction agreement, in addition to the cash paid to Genco for the Elgin, Gibson City, and Grand Tower energy center sale, Ameren will cause $70 million of cash to be retained at Genco in addition to the cash proceeds from the sale of put option assets of at least $133 million and an additional $15 million of cash to be retained at Marketing Company.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

Completion of the New AER divestiture to IPH is subject to the receipt of approvals from FERC and approval of certain license transfers by the FCC. On April 16, 2013, AER and Dynegy filed with FERC an application for approval of its divestiture of New AER and Genco’s sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Medina Valley. On July 26, 2013, FERC issued an order seeking additional information. In early August 2013, AER and Dynegy responded to FERC’s request for additional information. Several AER wholesale customers filed a protest with FERC regarding the application. Separately, as a condition to IPH’s obligation to complete the New AER transaction, the Illinois Pollution Control Board must approve the transfer to IPH of, or otherwise approve a variance in favor of IPH on the same terms as, AER’s variance of the Illinois MPS. In May 2013, AER and IPH filed a transfer request with the Illinois Pollution Control Board, which was subsequently denied by the board on procedural grounds. On July 22, 2013, IPH, AER, and Medina Valley, as current and future owners of the coal-fired energy centers, filed a request for a variance with the Illinois Pollution Control Board seeking the same relief as the existing AER variance. The Illinois Pollution Control Board has until late November 2013 to issue a decision. See Note 8 — Commitments and Contingencies for additional information. Ameren’s and IPH’s obligation to complete the transaction is also subject to other customary closing conditions, including the material accuracy of each company’s representations and warranties and the compliance, in all material respects, with each company’s covenants. The transaction agreement contains customary representations and warranties of Ameren and IPH, including representations and warranties of Ameren with respect to the business being sold. The transaction agreement also contains customary covenants of Ameren and IPH, including the covenant of Ameren that New AER will be operated in the ordinary course prior to the closing.

Ameren expects the closing of the New AER divestiture to IPH will occur in the fourth quarter of 2013. If the closing does not occur on or before March 14, 2014, subject to a one-month extension to obtain FERC approval, either party may elect to terminate the transaction agreement if the inability to close the transaction by such date is not the result of the failure of the terminating company to fulfill any of its obligations under the transaction agreement.

Amended Put Option Agreement, Asset Purchase Agreement and Guaranty

On March 28, 2012, Genco entered into a put option agreement with AERG, which gave Genco the option to sell to AERG all, but not less than all, of the Elgin, the Gibson City, and the Grand Tower gas-fired energy centers.

Prior to Ameren’s entry into the transaction agreement with IPH as discussed above, (i) the original put option agreement between Genco and AERG was novated and amended such that the rights and obligations of AERG under the agreement were assigned to and assumed by Medina Valley and (ii) Genco exercised its option under the amended put option agreement to sell the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Medina Valley. As a result, on March 14, 2013, Medina Valley paid to Genco an initial payment of $100 million, with an offset to “Deposit received from affiliate for pending asset sale,” in accordance with the terms of the amended put option agreement, asset purchase agreement, and transaction agreement with IPH. That deposit will remain on AER’s balance sheet until the assets and liabilities are transferred to Medina Valley. Genco advanced the initial payment amount received to the non-state-regulated subsidiaries money pool. In connection with the amended put option agreement, Ameren’s guaranty, dated March 28, 2012, was modified to replace all references to AERG with references to Medina Valley.

Pursuant to the amended put option agreement, Genco and Medina Valley entered into an asset purchase agreement, dated March 14, 2013. Genco and Medina Valley have engaged three appraisers to conduct a fair market valuation of the Elgin, Gibson City, and Grand Tower gas-fired

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

energy centers, whose valuations will be averaged and subject to adjustment at the closing of the asset purchase agreement to reflect the assets and liabilities associated with the Elgin, Gibson City, and Grand Tower gas-fired energy centers. At the closing, Genco will receive an additional amount equal to the greater of (i) $33 million, or (ii) the appraised value of the Elgin, Gibson City, and Grand Tower gas-fired energy centers less the initial payment of $100 million, for a total purchase price of at least $133 million, and Genco will sell and transfer to Medina Valley all of its rights in the Elgin, Gibson City, and Grand Tower gas-fired energy centers as a condition to the transaction agreement. If these gas-fired energy centers are subsequently sold by Medina Valley within two years of the asset purchase agreement closing, Medina Valley will pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the amounts previously paid to Genco. Ameren has commenced a sale process for these energy centers and expects a third-party sale will be completed during 2013. Should FERC approval not be obtained and the transfer of the Elgin, Gibson City, and Grand Tower energy centers cannot be completed, Genco will be required to return to Medina Valley the initial payment received in March 2013.

The asset purchase agreement contains customary representations, warranties and covenants of Genco and Medina Valley. The consummation of the transactions contemplated by the asset purchase agreement is subject to certain conditions, including the receipt of FERC approval and other customary conditions.

AER determined that the assets and liabilities associated with the Elgin, Gibson City, and Grand Tower gas-fired energy centers first qualified for held for sale presentation as of March 31, 2013, and continued to qualify as of June 30, 2013. As of December 31, 2012, these energy centers did not meet the criteria to be classified as held for sale as it was not probable at that time that they would be disposed of within one year. To enhance the comparability of these quarterly financial statements, AER has recast its December 31, 2012 balance sheet to reflect the presentation of the Elgin, Gibson City, and Grand Tower energy centers as held for sale at that date. The following table presents the components of assets and liabilities held for sale on AER’s consolidated balance sheet at June 30, 2013, and December 31, 2012:

As the assets and liabilities associated with the Elgin, Gibson City, and Grand Tower gas-fired energy centers continued to meet the held for sale criteria at June 30, 2013, AER evaluated whether any impairment existed by comparing the disposal group’s carrying value to the estimated fair value less cost to sell. The estimated fair value was determined by reference to the amended put option agreement, the asset purchase agreement with Medina Valley, and the transaction agreement with IPH, which AER believes approximates fair value. AER recorded a pretax charge to earnings of $207 million for the three months ended March 31, 2013, to reflect the impairment of the Elgin, Gibson City, and Grand Tower energy centers. The impairment charge decreased by $5 million during the three months ended June 30, 2013, as the carrying value of the Elgin, Gibson City and Grand Tower energy centers decreased primarily as a result of derivative market value

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

losses. As a result, AER recorded a cumulative pretax charge to earnings of $202 million for the six months ended June 30, 2013, to reflect the impairment of the Elgin, Gibson City, and Grand Tower gas-fired energy centers. The 2013 impairment recorded was primarily related to the Gibson City and Grand Tower energy centers as the Elgin energy center was previously impaired under held and used accounting guidance during the fourth quarter of 2012. The 2013 impairment charge to earnings was recorded as an impairment of “Property and Plant, net” within the components of current assets held for sale shown above as of June 30, 2013, and “Assets held for sale and impairment adjustments” in AER’s consolidated statement of operations and comprehensive income (loss) for the three and six months ended June 30, 2013. During the six months ended June 30, 2013, AER recorded a $79 million income tax benefit as a result of the impairment.

These assets and liabilities were measured at fair value on a nonrecurring basis with inputs that are classified as Level 3 within the fair value hierarchy.

Effective with AER’s conclusion in March 2013 that the Elgin, Gibson City, and Grand Tower energy centers met the criteria for held for sale presentation, AER suspended recording depreciation on these energy centers.

3.                            Short-term Debt and Liquidity

On November 14, 2012, the 2010 Genco Credit Agreement was terminated and not renewed. Should a financing need arise, sources of liquidity include available cash on hand, a return of money pool advances, and money pool borrowings at the discretion of Ameren. On March 14, 2013, Genco amended and exercised its option to sell its three natural gas-fired energy centers to Medina Valley for a sales price of at least $133 million. In March 2013, Genco received an initial payment of $100 million for the pending sale of its Elgin, Gibson City, and Grand Tower gas-fired energy centers to Medina Valley. Genco advanced the initial payment amount they received to the non-state-regulated subsidiary money pool. With the additional liquidity received through exercising the amended put option agreement, AER’s financing sources are estimated to be adequate to support its operations in 2013. See Note 2 - Assets Held for Sale for additional information regarding the amended put option agreement.

Money Pools

AER and its subsidiaries participate in money pool agreements that provide for certain short-term cash and working capital requirements. Separate money pools are maintained for Ameren’s utility and non-state-regulated entities. AMS is responsible for the operation and administration of the money pool agreements.

Non-state-regulated Subsidiaries

AER has the ability, subject to Ameren parent company authorization, to access funding from Ameren’s credit agreements and commercial paper programs through a non-state-regulated subsidiary money pool agreement. The total amount available to AER and other pool participants at any time is reduced by borrowings made by Ameren’s subsidiaries, but is increased to the extent that other pool participants advance surplus funds to the non-state-regulated subsidiary money pool or remit funds from other external sources. The non-state-regulated subsidiary money pool was established to coordinate and to provide short-term cash and working capital for Ameren’s non-state-regulated activities. Borrowers receiving a loan under the non-state-regulated subsidiary money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the non-state-regulated subsidiary money pool. These rates are based on the cost of funds used for

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

money pool advances. The average interest rate for borrowing under the non-state-regulated subsidiary money pool for the three and six months ended June 30, 2013, were 0.29% and 0.26%, respectively (2012 — 0.64% and 0.70%, respectively).

See Note 7 - Related Party Transactions for the amount of interest income and expense from the money pool arrangements for the three and six months ended June 30, 2013, and 2012.

Utility

AER (through AERG) may participate in the utility money pool only as a lender. There were no utility money pool advances by AERG during the six months ended June 30, 2013, and 2012.

4.                            Long-Term Debt

Indenture Provisions and Other Covenants

Genco is subject to Section 305(a) of the Federal Power Act, which makes it unlawful for any officer or director of a public utility, as defined in the Federal Power Act, to participate in the making or paying of any dividend from any funds “properly included in capital account.” The meaning of this limitation has never been clarified under the Federal Power Act or FERC regulations. However, FERC has consistently interpreted the provision to allow dividends to be paid as long as (1) the source of the dividends is clearly disclosed, (2) the dividends are not excessive, and (3) there is no self-dealing on the part of corporate officials.

Genco’s indenture includes provisions that require Genco to maintain certain interest coverage and/or debt-to-capital ratios in order for Genco to pay dividends, to make certain principal or interest payments on subordinated borrowings, to make loans to or investments in affiliates, or to incur additional external, third-party indebtedness. The following table summarizes these ratios for the 12 months ended and as of June 30, 2013:

(a)              As of the date of the restricted payment, as defined, the minimum ratio must have been achieved for the most recently ended four fiscal quarters and projected by management to be achieved for each of the subsequent four six-month periods. Investments in the non-state-regulated subsidiary money pool and repayments of non-state-regulated subsidiary money pool borrowings are not subject to this incurrence test.

(b)              Ratios must be computed on a pro forma basis considering the additional indebtedness to be incurred and the related interest expense.  Non-state-regulated subsidiary money pool borrowings are defined as permitted indebtedness and are not subject to these incurrence tests. Other borrowings from third-party external sources are included in the definition of indebtedness and to these incurrence tests.

Genco’s debt incurrence-related ratio restrictions under its indenture may be disregarded if both Moody’s and S&P reaffirm the ratings in place at the time of the debt incurrence after considering the additional indebtedness.

As shown in the table above, under the provisions of Genco’s indenture, Genco may not borrow additional funds from external, third-party sources if the interest coverage ratio is less than 2.5 or if the debt-to-capital ratio is greater than 60%. Beginning in the first quarter of 2013, Genco’s interest coverage ratio fell to a level less than the specified minimum level required for external borrowings, and is expected to remain less than this minimum level through at least 2015. As a result, Genco’s ability to borrow additional funds from external, third-party sources is restricted. Genco’s indenture

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

does not restrict intercompany borrowings from Ameren’s non-state-regulated subsidiary money pool. However, borrowings from the money pool are subject to Ameren’s control. If an intercompany financing need were to arise, borrowings from the non-state-regulated subsidiary money pool would be dependent on consideration by Ameren of the facts and circumstances existing at that time. During 2013, AER will seek to fund operations internally and therefore seek not to rely on financing from Ameren. Ameren’s transaction agreement with IPH requires Ameren to operate AER in the ordinary course of business prior to the closing. In December 2012, Ameren announced its intention to exit its Merchant Generation business as Ameren determined that business was no longer a core component of its future business strategy. If Ameren does not complete its divestiture of New AER, Ameren has indicated it expects to continue to reduce and ultimately eliminate its financial support of the Merchant Generation business until its ultimate exit from the business is complete. See Note 2 - Assets Held for Sale for additional information regarding the put option agreement.

In order for Genco to issue securities in the future, Genco will have to comply with all applicable requirements in effect at the time of any such issuances.

As shown in the table above, under the provisions of Genco’s indenture, dividends cannot be paid unless the actual interest coverage ratio for the most recently ended four fiscal quarters is greater than 1.75, which was not achieved during the four fiscal quarters ending June 30, 2013. Additionally, the interest coverage ratios projected by management for each of the subsequent four six-month periods must be greater than 1.75. Based on projections as of June 30, 2013, we do not believe that we will achieve the minimum interest coverage ratio necessary to pay dividends on our common stock for each of the subsequent four six-month periods ending December 31, 2013, June 20, 2014, December 31, 2014, or June 30, 2015. As a result, we were restricted from paying dividends as of June 30, 2013, and we expect to be unable to pay dividends through at least June 30, 2016.

Off-Balance-Sheet Arrangements

At June 30, 2013, AER did not have any off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business. Additionally, AER does not expect to engage in any significant off-balance-sheet financing arrangements in the near future.

5.                            Derivative Financial Instruments

AER uses derivatives principally to manage the risk of changes in market prices for natural gas, coal, diesel, and power. Such price fluctuations may cause the following:

·                       an unrealized appreciation or depreciation of AER’s contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;

·                       market values of coal and natural gas inventories that differ from the cost of those commodities in inventory; and

·                       actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays.

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

The derivatives that AER uses to hedge these risks are governed by AER’s risk management policies for forward contracts, futures, options, and swaps. AER’s net positions are continually assessed within its structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet AER’s requirements. Contracts AER enters into as part of its risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

The following table presents open gross commodity contract volumes by commodity type, as of June 30, 2013, and December 31, 2012:

(a)         Accrual contracts include commodity contracts that do not qualify as derivatives.  This includes contracts through December 2017, September 2035, and March 2014 for coal, power, and renewable energy credits, respectively, as of June 30, 2013.

(b)         Contracts through December 2016 for power as of June 30, 2013.

(c)          Contracts through December 2015, October 2016, April 2015, and June 2017 for coal, fuel oils, natural gas, and power, respectively, as of June 30, 2013. Includes amounts classified as held for sale.

(d)         Not applicable.

(e)          Fuel oils consist of heating oil, ultra-low sulfur diesel, and crude oil.

(f)           Includes intercompany eliminations.

Authoritative accounting guidance regarding derivative instruments requires that all contracts considered to be derivative instruments be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 6 - Fair Value Measurements for discussion of AER’s methods of assessing the fair value of derivative instruments. Many of AER’s physical contracts, such as purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense recorded in connection with NPNS contracts is recognized at the contract price upon physical delivery.

If AER determines that a contract meets the definition of a derivative and is not eligible for the NPNS exception, AER reviews the contract to determine if it qualifies for hedge accounting treatment. Contracts that qualify for cash flow hedge accounting treatment are recorded at fair value with changes in fair value charged or credited to accumulated OCI in the period in which the change occurs, to the extent the hedge is effective. To the extent the hedge is ineffective, the related changes in fair value are charged or credited to the statement of operations and comprehensive income (loss) in the period in which the change occurs. When the contract is settled or delivered, the net gain or loss is recorded in the statement of operations and comprehensive income (loss).

Certain derivative contracts are entered into on a regular basis as part of AER’s risk management program but do not qualify for, or AER does not choose to elect, the NPNS exception or hedge accounting treatment. Such contracts are recorded at fair value, with changes in fair value charged or credited to the statement of operations and comprehensive income (loss) in the period in which the change occurs.

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Table of Contents

Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

Authoritative accounting guidance permits companies to offset fair value amounts recognized for the right to reclaim cash collateral (a receivable) or the obligation to return cash collateral (a liability) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under the same master netting arrangement. AER did not elect to adopt this guidance for any eligible commodity contracts.

The following table presents the carrying value and balance sheet classification of all derivative instruments as of June 30, 2013, and December 31, 2012:

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)

The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments in accumulated OCI as of June 30, 2013, and December 31, 2012:

(a)         Represents net gains associated with power derivative contracts.  These contracts are a partial hedge of electricity price exposure through December 2016 as of June 30, 2013.  In light of market prices at June 30, 2013, net pretax unrealized gains of $21 million are expected to be reclassified into earnings during the next 12 months as the hedged transactions occur.  However, the actual amount reclassified from accumulated OCI could vary due to future changes in market prices.

(b)         Includes net losses associated with interest rate swaps at Genco.  The swaps were executed during the fourth quarter of 2007 as a partial hedge of interest rate risks associated with Genco’s April 2008 debt issuance.  The loss on the interest rate swaps is being amortized over a 10-year period that began in April 2008.  Over the next 12 months, $1.4 million of the loss will be amortized.

Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges and have nominal credit risk. In all other transactions, AER is exposed to credit risk. AER’s credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master trading and netting agreements, and reporting daily exposure to senior management.

AER believes that entering into master trading and netting agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. AER generally enters into the following master trading and netting agreements: (1) International Swaps and Derivatives Association Agreement, a standardized financial natural gas and electric contract; (2) the Master Power Purchase and Sale Agreement, created by the Edison Electric Institute and the National Energy Marketers Association, a standardized contract for the purchase and sale of wholesale power; and (3) North American Energy Standards Board, Inc.,  Agreement, a standardized contract for the purchase and sale of natural gas. These master trading and netting agreements allow the counterparties to net settle sale and purchase transactions.  Further, collateral requirements are calculated at a master trading and netting agreement level by counterparty.

Although AER had not previously elected to offset fair value amounts and collateral for derivative instruments executed with the same counterparty under the same master netting arrangement, authoritative accounting guidance, effective in the first quarter 2013, requires those amounts eligible to be offset to be presented both at the gross and net amounts. The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of June 30, 2013, and December 31, 2012:

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Ameren Energy Resources Company, LLC

(A subsidiary of Ameren Corporation)

Notes to Consolidated Financial Statements (Unaudited)

Three and Six Months Ended June 30, 2013, and 2012

(dollars in millions)