Exhibit 5.1

		SIDLEY AUSTIN LLP ONE SOUTH DEARBORN STREET         CHICAGO, IL 60603 (312) 853 7000 (312) 853 7036 FAX		BEIJING BRUSSELS CHICAGO DALLAS FRANKFURT GENEVA HONG KONG LONDON LOS ANGELES   FOUNDED 1866		NEW YORK PALO ALTO SAN FRANCISCO SHANGHAI SINGAPORE SYDNEY TOKYO WASHINGTON, D.C.

January 18, 2011

Commonwealth Edison Company

440 South LaSalle Street

Suite 3300

Chicago, Illinois 60605-1028

Ladies and Gentlemen:

We have acted as counsel to Commonwealth Edison Company, an Illinois corporation (the “Company”), in connection with the issuance and sale by the Company of $600,000,000 aggregate principal amount of First Mortgage 1.625% Bonds, Series 110, due January 15, 2014 (the “Bonds”), covered by the Registration Statement on Form S-3, No. 333-158920 (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (“SEC”) on April 30, 2009, under the Securities Act of 1933, as amended.

The Bonds were issued under the Company’s Mortgage (the “Mortgage”), dated July 1, 1923, as amended and supplemented by the Supplemental Indenture dated August 1, 1944 and subsequent supplemental indentures, including the Supplemental Indenture dated as of January 4, 2011, to BNY Mellon Trust Company of Illinois, as trustee, and D.G. Donovan, as co-trustee (collectively, the “Bond Trustees”), which Mortgage is governed by Illinois law, and sold by the Company pursuant to the Underwriting Agreement dated January 10, 2011 between the Company and Credit Suisse Securities (USA) LLC, J.P. Morgan Securities LLC and Morgan Stanley & Co. Incorporated, as representatives of the several underwriters named therein.

For the purpose of expressing the opinions in this letter, we have examined and relied upon a copy of the Registration Statement and the exhibits filed therewith. We have also examined originals, or copies of originals certified to our satisfaction, of such agreements, documents, certificates of officers of the Company and the Bond Trustees and statements of government officials and other instruments, and have examined such questions of law and have satisfied ourselves as to such matters of fact, as we have considered relevant and necessary as a basis for the opinions expressed in this letter. We have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals and the conformity with the original documents of all documents submitted to us as certified or photostatic copies or by facsimile or other means of electronic transmission or which we obtained from the Electronic Data Gathering, Analysis and Retrieval System (EDGAR) of the SEC or other internet sites through which documents filed with the SEC can be obtained. With respect to any instrument or agreement executed or to be executed by any party other than the

Sidley Austin LLP is a limited liability partnership practicing in affiliation with other Sidley Austin partnerships

Commonwealth Edison Company

January 18, 2011

Page 2

Company, we have assumed, to the extent relevant to the opinions set forth herein, that (i) such other party (if not a natural person) was duly organized or formed, as the case may be, and was at all relevant times and is validly existing and in good standing under the laws of its jurisdiction of organization or formation, as the case may be, and (ii) such other party had at all relevant times and has full right, power and authority to execute, deliver and perform its obligations under each instrument or agreement to which it is a party and each such instrument or agreement has been duly authorized (if applicable), executed and delivered by, and was at all relevant times and is a valid, binding and enforceable agreement or obligation, as the case may be, of, such other party.

Based on the foregoing, and subject to the limitations hereinafter set forth, we are of the opinion that:

Our opinion in paragraph 2 with respect to the enforceability of the Bonds is subject to:

This letter is limited to the federal laws of the United States of America and the laws of the State of Illinois.

We do not find it necessary for the purposes of this letter to cover, and accordingly we express no opinion as to, the application of the securities or blue sky laws of the various states or the District of Columbia to the Bonds.

Commonwealth Edison Company

January 18, 2011

Page 3

We hereby consent to the filing of this letter as Exhibit 5-1-2 to the Registration Statement and the references to our firm included in or made a part of the Registration Statement.