SUPPLEMENTAL INDENTURE

Exhibit 4.2

This Supplemental Indenture, dated as of March 1, 2007, made and entered into by and between CENTRAL ILLINOIS PUBLIC SERVICE COMPANY, a corporation organized and existing under the laws of the State of Illinois (hereinafter commonly referred to as the “ Company ”), and U.S. BANK NATIONAL ASSOCIATION (formerly U.S. Bank Trust National Association, formerly First Trust National Association, formerly First Trust of Illinois, National Association, successor trustee to Bank of America Illinois, formerly Continental Bank, formerly Continental Bank, National Association and formerly Continental Illinois National Bank and Trust Company of Chicago), a national banking association having its office or place of business in the City of Chicago, Cook County, State of Illinois (hereinafter commonly referred to as the “ Trustee ”), and Richard Prokosch (successor Co-Trustee), of the City of Oakdale, Washington County, State of Minnesota, as Trustees under the Indenture of Mortgage or Deed of Trust dated October 1, 1941, heretofore executed and delivered by the Company to Continental Illinois National Bank and Trust Company of Chicago and Edmond B. Stofft, as Trustees, as amended by the Supplemental Indentures dated, respectively, September 1, 1947, January 1, 1949, February 1, 1952, September 1, 1952, June 1, 1954, February 1, 1958, January 1, 1959, May 1, 1963, May 1, 1964, June 1, 1965, May 1, 1967, April 1, 1970, April 1, 1971, September 1, 1971, May 1, 1972, December 1, 1973, March 1, 1974, April 1, 1975, October 1, 1976, November 1, 1976, October 1, 1978, August 1, 1979, February 1, 1980, February 1, 1986, May 15, 1992, July 1, 1992, September 15, 1992, April 1, 1993, June 1, 1995, March 15, 1997, June 1, 1997, December 1, 1998, June 1, 2001, October 1, 2004, June 1, 2006 and August 1, 2006 heretofore executed and delivered by the Company to the Trustees under said Indenture of Mortgage or Deed of Trust dated October 1, 1941; said Indenture of Mortgage or Deed of Trust dated October 1, 1941, as amended by said Supplemental Indentures, being hereinafter sometimes referred to as the “ Indenture ”; and said U.S. Bank National Association and Richard Prokosch (successor Co-Trustee), of the City of Oakdale, Washington County, State of Minnesota, as Trustees, as such Trustees, being hereinafter sometimes referred to as the “ Trustees ” or the “ Trustees under the Indenture ”;

WITNESSETH:

WHEREAS, the Company has entered into a Credit Agreement, dated as of February 9, 2007 (as amended or otherwise modified from time to time, the “ Credit Agreement ”) by and among the Company, Central Illinois Light Company, Illinois Power Company, AmerenEnergy Resources Generating Company and CILCORP Inc., as borrowers, the lenders from time to time party thereto (the “ Lenders ”) and JPMorgan Chase Bank, N.A., as agent (in such capacity, the “ Agent ”) for the Lenders, providing for the making of certain financial accommodations thereunder to the Company, and pursuant to such Credit Agreement, the Company has agreed to issue to the Agent, as evidence of and security for the Obligations (as such term is defined in the Credit Agreement) of the Company (the “ Company Obligations ”), a new series of bonds under the Indenture;

WHEREAS, for such purposes, the Company has determined, by resolutions duly adopted by its Board of Directors, to issue bonds of an additional series under and to be secured by the Indenture, as hereby amended, to be known and designated as First Mortgage Bonds, 2007 Credit Agreement Series (hereinafter sometimes referred to as the “ bonds of 2007 Credit Agreement Series ” or the “ bonds of said Series ”), and the bonds of said Series shall be authorized, authenticated and issued only as registered bonds without coupons, and to execute

 and deliver this supplemental indenture, pursuant to the provisions of Article I, as amended, Section 6 of Article II and Article XVI of the Indenture, for the purpose of (1) creating and authorizing not to exceed $135,000,000 aggregate principal amount of bonds of 2007 Credit Agreement Series and setting forth the form, terms, provisions and characteristics thereof, and (2) modifying or amending certain provisions of the Indenture in the particulars and to the extent hereinafter specifically provided; and

WHEREAS, the bonds of 2007 Credit Agreement Series   shall be issued to the Agent as evidence of and security for the Company Obligations under the Credit Agreement;

WHEREAS, the execution and delivery by the Company of this supplemental indenture have been duly authorized by the Board of Directors of the Company; and the Company has requested, and hereby requests, the Trustees to enter into and join with the Company in the execution and delivery of this supplemental indenture; and

WHEREAS, the bonds of 2007 Credit Agreement Series are to be authorized, authenticated and issued only in the form of registered bonds without coupons, and the bonds of 2007 Credit Agreement Series and the certificate of the Trustee thereon shall be substantially in the following form, to wit:

[FORM OF BOND]

Illinois Commerce Commission

Identification No.: Ill. C.C. No. ____

Notwithstanding any provisions hereof or in the Indenture

this Bond is not assignable or transferable except to a successor Agent appointed in

accordance with the Credit Agreement, dated

as of February 9, 2007, hereinafter referred to.

Central Illinois Public Service Company

First Mortgage Bond, 2007 Credit Agreement Series

CENTRAL ILLINOIS PUBLIC SERVICE COMPANY, an Illinois corporation (hereinafter referred to as the “ Company ”), for value received, hereby promises to pay to the Registered Owner specified above, as agent (in such capacity, the “Agent”) for the Lenders (as defined below) under the Credit Agreement, dated as of February 9, 2007, by and among the Company, Central Illinois Light Company, Illinois Power Company, AmerenEnergy Resources Generating Company and CILCORP Inc., as borrowers, the lenders from time to time party thereto (the “ Lenders ”) and JPMorgan Chase Bank, N.A., as agent (as amended or otherwise modified from time to time, the “ Credit Agreement ”), or registered assigns, the Principal Amount

specified above or such lesser principal amount as shall be equal to the Borrower Credit Exposure (as defined in the Credit Agreement) of the Company outstanding on the Maturity Date (having at any time the meaning such term has at such time under the Credit Agreement) of the Company, but not in excess of the Principal Amount of this bond, and to pay interest thereon at the Interest Rate (as defined below) until the principal hereof is paid or duly made available for payment on the Maturity Date or in the event of redemption of this bond, until the redemption date.

Interest on this bond shall be payable on each Interest Payment Date (as defined below), commencing on the first Interest Payment Date next succeeding the date of this bond. If the Maturity Date falls on a day which is not a Business Day, as defined below, principal and any interest and/or fees payable with respect to the Maturity Date will be paid on the next succeeding Business Day. The interest payable, and punctually paid or duly provided for, on any Interest Payment Date will, subject to certain exceptions provided in the Supplemental Indenture dated as of March 1, 2007, hereinafter referred to, be paid to the person in whose name this bond (or one or more predecessor bonds) is registered at the close of business on the Record Date (as defined below); provided, however, that interest payable on the Maturity Date will be payable to the person to whom the principal hereof shall be payable. Should the Company default in the payment of interest (“Defaulted Interest”), the Defaulted Interest shall be paid to the person in whose name this bond is registered on the Record Date to be established by the Trustee for payment of such Defaulted Interest. As used herein, (A) “Business Day” shall have the meaning assigned thereto in the Credit Agreement; (B) “Interest Payment Date” shall mean each date on which Company Obligations constituting interest and/or fees are due and payable from time to time pursuant to the Credit Agreement; (C) “Interest Rate” shall mean a rate of interest per annum, adjusted as necessary, to result in an interest payment equal to the aggregate amount of Company Obligations constituting interest and fees of the Company due under the Credit Agreement on the applicable Interest Payment Date; and (D) “Record Date” with respect to any Interest Payment Date shall mean the day (whether or not a Business Day) immediately next preceding such Interest Payment Date.

Both the principal of and the interest on this bond shall be payable in immediately available funds at the office or agency of the Trustee, in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts.

This bond is one of the bonds issued and to be issued from time to time under and in accordance with and all secured by the indenture of mortgage or deed of trust dated October 1, 1941, executed and delivered by the Company to U.S. Bank National Association (formerly U.S. Bank Trust National Association, formerly First Trust National Association, formerly First Trust of Illinois, National Association, successor trustee to Bank of America Illinois, formerly Continental Bank, formerly Continental Bank, National Association and formerly Continental Illinois National Bank and Trust Company of Chicago and hereinafter referred to as the “ Trustee ”) and Edmond B. Stofft, as Trustees, and the various indentures supplemental thereto, including the Supplemental Indenture pursuant to which $135,000,000 in aggregate principal amount of the First Mortgage Bonds, 2007 Credit Agreement Series (the “ 2007 Credit Agreement Series Bonds ”) are authorized, each executed and delivered by the Company to the Trustees under said indenture of mortgage or deed of trust dated October 1, 1941, prior to the authentication of this bond (said indenture of mortgage or deed of trust and said supplemental

 indentures being hereinafter referred to, collectively, as the “ Indenture ”); and said U.S. Bank National Association and Richard Prokosch (successor Co-Trustee), of the City of Oakdale, Washington County, State of Minnesota, as Trustees (successor Co-Trustee) being now the Trustees under the Indenture. Reference to the Indenture and to all supplemental indentures, if any, hereafter executed pursuant to the Indenture is hereby made for a description of the property mortgaged and pledged, the nature and extent of the security and the rights of the holders and Registered Owners of said bonds and of the Trustees and of the Company in respect of such security. By the terms of the Indenture the bonds to be secured thereby are issuable in series, which may vary as to date, amount, date of maturity, rate of interest, redemption provisions, medium of payment and in other respects as in the Indenture provided.

The 2007 Credit Agreement Series Bonds   are to be issued and delivered to the Agent in order to evidence and secure the obligations of the Company under the Credit Agreement to make payments to the Lenders under the Credit Agreement and to provide the Lenders the benefit of the lien of the Indenture with respect to the 2007 Credit Agreement Series Bonds.

The obligation of the Company to make payments with respect to principal under the Credit Agreement shall not give rise to an obligation to pay principal of the 2007 Credit Agreement Series Bonds except on the Maturity Date of the Company or upon redemption hereof. If at any time any permanent reduction of the Borrower Sublimit (as defined in the Credit Agreement) of the Company or the Borrower Credit Exposure (as defined in the Credit Agreement) of the Company shall result in the principal of the 2007 Credit Agreement Series Bonds being greater than the greater of the Borrower Sublimit and the Borrower Credit Exposure, a payment obligation with respect to the principal of the 2007 Credit Agreement Series Bonds in the amount of such excess shall be deemed discharged upon the effectiveness of such permanent reduction. No payment of principal under the Credit Agreement shall reduce the principal amount of the   2007 Credit Agreement Series Bonds to an amount less than the greater of the Borrower Sublimit and the Borrower Credit Exposure.

The obligation of the Company to make payments with respect to the interest on the 2007   Credit Agreement Series Bonds shall be fully or partially, as the case may be, satisfied and discharged to the extent that, at the time that any such payment shall be due, the then due interest and/or fees of the Company under the Credit Agreement shall have been fully or partially paid. Satisfaction of any obligation to the extent that payment is made with respect to the interest and/or fees of the Company under the Credit Agreement means that if any payment is made on the interest and/or fees of the Company under the Credit Agreement, a corresponding payment obligation with respect to the interest on the 2007 Credit Agreement Series Bonds shall be deemed discharged in the same amount as such payment made on the interest and/or fees of the Company under the Credit Agreement.

The Trustee may at any time and all times conclusively assume that the obligation of the Company to make payments with respect to the principal of and interest on the 2007 Credit Agreement Series Bonds, so far as such payments at the time have become due, has been fully satisfied and discharged pursuant to the foregoing paragraphs unless and until the Trustee shall have received a written notice from the Agent stating (i) that timely payment of principal of or interest on the 2007 Credit Agreement Series Bonds has not been made, (ii) that the Company is

in arrears as to the payments required to be made by it to the Agent in connection with the Company Obligations pursuant to the Credit Agreement, and (iii) the amount of the arrearage.

This bond is not redeemable except upon written demand of the Agent following the occurrence of a Default by the Company under the Credit Agreement and the acceleration of the Company Obligations, as provided under the Credit Agreement.

In case of certain events of default specified in the Indenture, the principal of this bond may be declared or may become due and payable in the manner and with the effect provided in the Indenture. No recourse shall be had for the payment of the principal of or interest on this bond, or for any claim based hereon, or otherwise in respect hereof or of the Indenture or any indenture supplemental thereto, to or against any incorporator, stockholder, officer or director, past, present or future, of the Company, or of any predecessor or successor corporation, either directly or through the Company, or such predecessor or successor corporation, under any constitution or statute or rule of law, or by the enforcement of any assessment, penalty, or otherwise, all such liability of incorporators, stockholders, directors and officers being waived and released by the Registered Owner hereof by the acceptance of this bond and being likewise waived and released by the terms of the Indenture.

This bond shall not be assignable or transferable except to a successor Agent appointed in accordance with the Credit Agreement. This bond is exchangeable by the Registered Owner hereof, in person or by attorney duly authorized, at the principal office or place of business of the Trustee under the Indenture, upon the surrender and cancellation of this bond and the payment of any stamp tax or other governmental charge, and upon any such exchange a new registered bond or bonds without coupons,