Unocal [Logo appears here]

                                           Samuel H. Gillespie III
                                           Senior Vice President
                                           Chief Legal Officer & General Counsel
                                           Tel:  310.726.7783
                                           Fax:  310.726.7815
                                           sgillespie@unocal.com

                                       June 7, 2004



Unocal Corporation
2141 Rosecrans Avenue
Suite 4000
El Segundo, California  90245

         Re:      Unocal Corporation:
                  2004 Directors' Deferred Compensation and
                  Restricted Stock Unit Award Plan
                  ---------------------------------

Ladies and Gentlemen:

         I am the Senior Vice President, Chief Legal Officer and General Counsel
of Unocal Corporation, a Delaware corporation ("Unocal"), and as such I, and
                                                ------
attorneys working for me, have acted on behalf of Unocal as counsel in
connection with the preparation of the Registration Statement on Form S-8 (the
"Registration Statement"), which Unocal proposes to file with the Securities and
 ----------------------
Exchange Commission. The Registration Statement relates to the registration
under the Securities Act of 1933, as amended (the "Securities Act"), of
                                                   --------------
500,000 shares (the "Shares") of Unocal Common Stock, par value $1.00 per
share, issued under the 2004 Directors' Deferred Compensation and
Restricted Stock Unit Award Plan (the "Plan").
                                       ----

         This opinion is rendered in accordance with the requirements of Item
601(b)(5) of Regulation S-K of the Securities and Exchange Commission.

         I, or such attorneys working for me, have examined or considered such
matters of law and fact and such corporate records, certificates and other
documents as I, or they, have deemed necessary, including, but not limited to:
the Certificate of Incorporation and Bylaws of Unocal, as amended to date; the
Rights Agreement, dated as of January 5, 2000, between Unocal and Mellon
Investor Services, L.L.C., as Rights Agent, as amended by Amendment to Rights
Agreement dated as of March 27, 2002; certain resolutions adopted by the Board
of Directors of Unocal at a meeting duly called and held on February 10, 2004
and by the stockholders at a meeting duly called and held on May 24, 2004; the
Plan; and certificates and other information obtained from public officials and
officers and employees of Unocal and its subsidiaries. In the course of such
examination, I, and such attorneys working for me, have assumed the genuineness
of all signatures, the authenticity of all documents submitted as originals and
the conformity to the originals of all documents submitted as certified,
photostatic or conformed copies. I have


Unocal Corporation  2141 Rosecrans Avenue, Suite 4000, El Segundo, CA 90245


relied, as to certain legal matters, on the advice of such attorneys working
for me who are more familiar with such matters.

         I am licensed to practice law in the state of New York and am familiar
with the Delaware General Corporation Law, the Delaware Constitution and
reported judicial decisions interpreting these laws. Therefore, the following
opinions are limited to the Delaware General Corporation Law and the Delaware
Constitution and reported judicial decisions interpreting these laws, and the
federal laws of the United States, to the exclusion of all other jurisdictions.

         Based on and subject to the foregoing, I am of the opinion that the
Shares were duly authorized for issuance by the Board of Directors of Unocal and
that, when one or more certificates evidencing the Shares has been duly
countersigned by the transfer agent and registrar for the Common Stock, and the
Shares have been issued to and paid for in accordance with the Plan and
procedures established between the transfer agent and registrar for the Common
Stock and The Depository Trust Company, the shares will be validly issued, fully
paid and non-assessable.

         I hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an Exhibit to the Registration Statement and to the
reference to me under the caption "Interests of Named Experts and Counsel"
therein. In giving this consent, I do not thereby admit that I am included in
the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations of the Commission promulgated
thereunder.

                                       Sincerely,

                                       /S/ Samuel H. Gillespie III