UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (Date of earliest event reported):

October 4, 2011

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Reference is made to Note 9 – Commitments and Contingencies in our financial statements under Part I, Item 1. Financial Statements and Results of Operations and Outlook under Part I, Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations, each in the Form 10-Q for the quarterly period ended June 30, 2011, of registrant Ameren Corporation (“Ameren”) and its registrant subsidiary Ameren Energy Generating Company (“Genco”), for a discussion of the compliance options with respect to the Cross-State Air Pollution Rule (“CSAPR”) for Genco’s Meredosia and Hutsonville energy centers.

On October 4, 2011, Ameren Energy Resources Company, LLC (“AER”), the holding company for Ameren’s merchant generation business, announced that a total of four currently operating units at Genco’s Meredosia and Hutsonville energy centers will cease operating at the end of 2011. The net generating capacity of Meredosia Energy Center is 369 megawatts—including one 203-megawatt, coal-fired unit and one 166-megawatt, oil-fired unit. The Hutsonville Energy Center has two coal-fired units with a net generating capacity of 151 megawatts.

The shutdown of these units will result in the elimination of 90 positions. As a result of these shutdowns, each of Ameren and Genco expects to record a charge to earnings in the third quarter of 2011, representing (1) a non-cash impairment of plant book value ($26 million before taxes), (2) a non-cash impairment of materials and supplies ($5 million before taxes), and (3) estimated future cash severance costs ($4 million before taxes). Ameren expects to receive cash tax benefits of approximately $22 million (Genco - $33 million) as a result of the shutdown of these units. Previously recorded asset retirement obligations for ash pond closures, and river structure and asbestos removals for these energy centers were $38 million with cash expenditures expected over the next ten years along with associated cash tax benefits of $16 million.

The shutdown of these units is primarily the result of the expected cost of complying with the CSAPR, which was issued in July 2011 by the U.S. Environmental Protection Agency. Genco determined that CSAPR compliance options for these four units were uneconomical. Another factor driving the shutdown of these facilities is a lack of a multi-year capacity market managed by the Midwest Independent Transmission System Operator, Inc. (MISO), without which Genco is not positioned to make the substantial investment for environmental controls that would be required to keep these units in service.

Forward-Looking Statements

Statements in this report not based on historical facts are considered “forward-looking” and, accordingly, involve risks and uncertainties that could cause actual results to differ materially from those discussed. Although such forward-looking statements have been made in good faith and are based on reasonable assumptions, there is no assurance that the expected results will be achieved. These statements include (without limitation) statements as to future expectations, beliefs, plans, strategies, objectives, events, conditions, and financial performance. In connection with the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995, we are providing this cautionary statement to identify important factors that could cause actual results to differ materially from those anticipated. The following factors, in addition to those discussed under Risk Factors in Ameren’s and Genco’s Form 10-K for the year ended December 31, 2010, and elsewhere in this report and in our other filings with the Securities and Exchange Commission, could cause actual results to differ materially from management expectations suggested in such forward-looking statements:

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Given these uncertainties, undue reliance should not be placed on these forward-looking statements. Except to the extent required by the federal securities laws, we undertake no obligation to update or revise publicly any forward-looking statements to reflect new information or future events.

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This combined Form 8-K is being filed separately by Ameren Corporation and Ameren Energy Generating Company (each a “registrant”). Information contained herein relating to any individual registrant has been filed by such registrant on its own behalf. No registrant makes any representation as to information relating to any other registrant.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, each registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized. The signature for each undersigned company shall be deemed to relate only to matters having reference to such company or its subsidiaries.

Date: October 7, 2011

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