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                                                                     EXHIBIT 5.1

                       [MAYER, BROWN & PLATT LETTERHEAD]

                                August 29, 2001


Devon Energy Corporation
20 North Broadway, Suite 1500
Oklahoma City, Oklahoma 73102

Re:  Merger of Mitchell Energy & Development Corp.
     with and into Devon NewCo Corporation, a wholly
     owned subsidiary of Devon Energy Corporation

Ladies and Gentlemen:

     We have acted as special counsel to Devon Energy Corporation, a Delaware
corporation ("Devon"), in connection with the corporate proceedings taken and
to be taken relating to the merger of Mitchell Energy & Development Corp., a
Texas corporation ("Mitchell"), with and into Devon NewCo Corporation, a wholly
owned subsidiary of Devon ("Merger Sub"), with Merger Sub being the surviving
corporation (the "Merger"), and the conversion of each share of Mitchell Class A
common stock, par value $0.10 per share, issued and outstanding at the
effective time of the Merger into (i) the right to receive $31.00 in cash and
(ii) 0.585 of a share of Devon common stock, par value $0.10 per share ("Devon
Common Stock"). We have also participated in the preparation and filing with
the Securities and Exchange Commission under the Securities Act of 1933, as
amended, of a registration statement on Form S-4 (the "Registration Statement")
relating to the Merger. In this connection, we have examined such corporate and
other records, instruments, certificates and documents as we considered
necessary to enable us to express this opinion.

     Based on the foregoing, it is our opinion that the Devon Common Stock has
been duly and validly authorized by all necessary action on the part of Devon
and when issued pursuant to the terms of the Agreement and Plan of Merger,
dated as of August 13, 2001, by and among Devon, Merger Sub and Mitchell, will
be validly issued, fully paid and non-assessable by Devon.

     We consent to the filing of this opinion as an exhibit to the Registration
Statement and to the reference to us under the caption "Additional
Information--Legal Matters" therein.

                                               Very truly yours,


                                               /s/ MAYER, BROWN & PLATT

                                               Mayer, Brown & Platt