Post-Effective Amendment No. 1 to Form S-4

Large accelerated filer

Accelerated filer

Non-accelerated filer

Smaller reporting company

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale to the public: This post-effective amendment deregisters all of the securities that were unsold under the registration statement as of the date hereof.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box. ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box, and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

Indicate by checkmark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer”, “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

This Post-Effective Amendment No. 1 on Form S-4 (Registration No. 333-156606) shall become effective in accordance with Section 8(c) of the Securities Act of 1933 on such date as the Securities and Exchange Commission, acting pursuant to Section 8(c), may determine.

On January 7, 2009, Illinois Power Company (“IP”) filed a Registration Statement on Form S-4 (Registration No. 333-156606), as amended by Amendment No. 1 thereto filed on February 17, 2009 (collectively, the “Registration Statement”), with the United States Securities and Exchange Commission, which registered $400,000,000 in aggregate principal amount of its 9.75% Senior Secured Notes due 2018 to be issued in exchange for its $400,000,000 in aggregate principal amount of its unregistered 9.75% Senior Secured Notes due 2018.

On October 1, 2010, Central Illinois Public Service Company (“CIPS”), Central Illinois Light Company (“CILCO”) and IP completed a merger whereby CILCO and IP merged with and into CIPS, with CIPS as the surviving entity (the “Merger”), pursuant to the terms of the agreement and plan of merger, dated as of April 13, 2010, among CIPS, CILCO and IP. Upon consummation of the Merger, CIPS’ name was changed to “Ameren Illinois Company,” and the separate legal existence of IP was terminated. Prior to the Merger, each of CIPS and IP was a registrant subsidiary of Ameren Corporation. As a result of the Merger, IP’s offering pursuant to the Registration Statement has been terminated. Therefore, IP hereby removes from registration the securities of IP registered but unsold under the Registration Statement as of the date hereof.

Pursuant to the requirements of the Securities Act of 1933 and Rule 478 thereunder, Ameren Illinois Company (as successor to Illinois Power Company) has duly caused this Post-Effective Amendment No. 1 to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of St. Louis, State of Missouri, on the 21st day of December, 2010.


AMEREN ILLINOIS COMPANY (AS SUCCESSOR TO ILLINOIS POWER COMPANY) (REGISTRANT)

By:		/s/ MARTIN J. LYONS, JR.
		Martin J. Lyons, Jr. Senior Vice President and Chief Financial Officer

Note: No other person is required to sign this Post-Effective Amendment No. 1 to the Registration Statement on behalf of Ameren Illinois Company (as successor to Illinois Power Company) in reliance upon Rule 478 under the Securities Act of 1933.