SUPPLEMENTAL INDENTURE

EXHIBIT 4.1

This instrument was prepared by,

and when recorded should be

returned to:

Richard W. Astle

Sidley Austin LLP

One South Dearborn Street

Chicago, Illinois 60603

S UPPLEMENTAL I NDENTURE

Dated as of March 10, 2008

C OMMONWEALTH E DISON C OMPANY

to

BNY M IDWEST T RUST C OMPANY

and

D.G. D ONOVAN

Trustees Under Mortgage Dated July 1, 1923,

and Certain

Indentures Supplemental Thereto

Providing for Issuance of

FIRST MORTGAGE 5.80% BONDS, SERIES 108

Due March 15, 2018

T HIS S UPPLEMENTAL I NDENTURE , dated as of March 10, 2008, between C OMMONWEALTH E DISON C OMPANY , a corporation organized and existing under the laws of the State of Illinois (hereinafter called the “ Company ”) having an address at 440 South LaSalle Street, Suite 3300, Chicago, Illinois 60605, party of the first part, BNY M IDWEST T RUST C OMPANY , a trust company organized and existing under the laws of the State of Illinois having an address at 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602, and D.G. D ONOVAN , an individual having an address at 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602, as Trustee and Co-Trustee, respectively, under the Mortgage of the Company dated July 1, 1923, as amended and supplemented by Supplemental Indenture dated August 1, 1944 and the subsequent supplemental indentures hereinafter mentioned, parties of the second part (said Trustee being hereinafter called the “ Trustee ”, the Trustee and said Co-Trustee being hereinafter together called the “ Trustees ”, and said Mortgage dated July 1, 1923, as amended and supplemented by said Supplemental Indenture dated August 1, 1944 and subsequent supplemental indentures, being hereinafter called the “ Mortgage ”),

W I T N E S S E T H:

WHEREAS, the Company duly executed and delivered the Mortgage to provide for the issue of, and to secure, its bonds, issuable in series and without limit as to principal amount except as provided in the Mortgage; and

WHEREAS, the Company from time to time has executed and delivered supplemental indentures to the Mortgage to provide for (i) the creation of additional series of bonds secured by the Mortgage, (ii) the amendment of certain of the terms and provisions of the Mortgage and (iii) the confirmation of the lien of the Mortgage upon property of the Company, such supplemental indentures that are currently effective and the respective dates, parties thereto and purposes thereof, being as follows:

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WHEREAS, the respective designations, maturity dates and stated principal amounts of the bonds of each series presently outstanding under, and secured by, the Mortgage and the several supplemental indentures above referred to, are as follows:

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WHEREAS, the Mortgage provides for the issuance from time to time thereunder, in series, of bonds of the Company for the purposes and subject to the limitations therein specified; and

WHEREAS, the Company desires, by this Supplemental Indenture, to create an additional series of bonds to be issuable under the Mortgage, such bonds to be designated “First Mortgage 5.80% Bonds, Series 108 (hereinafter called the “ bonds of Series 108 ”) and the terms and provisions to be contained in the bonds of Series 108 or to be otherwise applicable thereto to be as set forth in this Supplemental Indenture; and

WHEREAS, the bonds of Series 108 and the Trustee’s certificate to be endorsed thereon shall be substantially in the form of the General Form of Registered Bond Without Coupons and the form of the General Form of Trustee’s Certificate set forth in Section 3.05 of the Supplemental Indenture dated August 1, 1944 to the Mortgage with such appropriate insertions, omissions and variations in order to express the designation, date, maturity date, annual interest rate, record dates for, and dates of, payment of interest, denominations, terms of redemption and redemption prices, and other terms and characteristics authorized or permitted by the Mortgage or not inconsistent therewith; and

WHEREAS, the Company is legally empowered and has been duly authorized by the necessary corporate action and by an order or orders of the Illinois Commerce Commission to make, execute and deliver this Supplemental Indenture, and to create, as an additional series of bonds of the Company, the bonds of Series 108, and all acts and things whatsoever necessary to make this Supplemental Indenture, when executed and delivered by the Company and the Trustees, a valid, binding and legal instrument, and to make the bonds of Series 108, when authenticated by the Trustee and issued as in the Mortgage and in this Supplemental Indenture provided, the valid, binding and legal obligations of the Company, entitled in all respects to the security of the Mortgage, as amended and supplemented, have been done and performed;

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NOW, THEREFORE, in consideration of the premises and of the sum of one dollar duly paid by the Trustees to the Company, and for other good and valuable consideration, the receipt of which is hereby acknowledged, the parties hereto do hereby agree as follows:

SECTION 1. Designation and Issuance of Bonds of Series 108 . The bonds of Series 108 shall, as hereinbefore recited, be designated as the Company’s “First Mortgage 5.80% Bonds, Series 108.” Subject to the provisions of the Mortgage, the bonds of Series 108 shall be issuable without limitation as to the aggregate principal amount thereof.

SECTION 2. Form, Date, Maturity Date, Interest Rate and Interest Payment Dates of Bonds of Series 108 . (a) The definitive bonds of Series 108 shall be in engraved, lithographed, printed or typewritten form and shall be registered bonds without coupons; and such bonds and the Trustee’s certificate to be endorsed thereon shall be substantially in the forms hereinbefore recited, respectively. The bonds of Series 108 shall be dated as provided in Section 3.01 of the