UNION OIL CO OF CALIFORNIA
424B5, 1998-05-04
PETROLEUM REFINING
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<PAGE>
                                          Filed Pursuant to Rule 424(b)(5)
                                          Registration No.:033-54861
                                                           033-54861-01
PROSPECTUS SUPPLEMENT
(TO PROSPECTUS DATED FEBRUARY 3, 1995)
 
                                  $300,000,000
 
                                     [LOGO]
                        UNION OIL COMPANY OF CALIFORNIA
                       $100,000,000 6 1/2% NOTES DUE 2008
                      $200,000,000 7% DEBENTURES DUE 2028
 
       PAYMENT OF PRINCIPAL, INTEREST AND PREMIUM, IF ANY, GUARANTEED BY
                               UNOCAL CORPORATION
                                    --------
 
                     INTEREST PAYABLE MAY 1 AND NOVEMBER 1
                              -------------------
 
THE NOTES WILL MATURE ON MAY 1, 2008, AND THE DEBENTURES WILL MATURE ON MAY 1,
2028. THE NOTES AND THE DEBENTURES (COLLECTIVELY, THE "OFFERED SECURITIES")
 WILL BE REDEEMABLE, IN WHOLE OR FROM TIME TO TIME IN PART, AT THE OPTION OF
 THE COMPANY AT ANY TIME AT A REDEMPTION PRICE EQUAL TO THE GREATER OF (A)
  100% OF THE PRINCIPAL AMOUNTS OF THE OFFERED SECURITIES TO BE REDEEMED OR
  (B) THE SUM OF THE PRESENT VALUES OF THE REMAINING SCHEDULED PAYMENTS OF
   PRINCIPAL AND INTEREST THEREON (EXCLUSIVE OF INTEREST ACCRUED TO THE DATE
   OF REDEMPTION) DISCOUNTED TO THE DATE OF REDEMPTION ON A SEMI-ANNUAL
    BASIS (ASSUMING A 360-DAY YEAR CONSISTING OF TWELVE 30-DAY MONTHS) AT
     THE TREASURY RATE (AS DEFINED HEREIN) PLUS 10 BASIS POINTS (IN THE
     CASE OF THE NOTES) OR 20 BASIS POINTS (IN THE CASE OF THE DEBENTURES),
     PLUS, IN EITHER CASE, ACCRUED AND UNPAID INTEREST ON THE PRINCIPAL
     AMOUNT BEING REDEEMED TO THE DATE OF REDEMPTION. THE OFFERED
      SECURITIES WILL NOT BE SUBJECT TO ANY SINKING FUND. THE OFFERED
      SECURITIES WILL BE REPRESENTED BY GLOBAL SECURITIES REGISTERED IN
       THE NAME OF THE DEPOSITORY TRUST COMPANY (THE "DEPOSITARY") OR ITS
       NOMINEES. INTERESTS IN SUCH GLOBAL SECURITIES WILL BE SHOWN ON,
        AND TRANSFERS THEREOF WILL BE EFFECTED ONLY THROUGH, RECORDS
        MAINTAINED BY THE DEPOSITARY AND ITS PARTICIPANTS. EXCEPT AS
        DESCRIBED HEREIN, OFFERED SECURITIES IN DEFINITIVE FORM WILL NOT
            BE ISSUED. SEE "DESCRIPTION OF THE OFFERED SECURITIES."
 
                            ------------------------
 
THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
 EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE SECURITIES
   AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION PASSED UPON
     THE ACCURACY OR ADEQUACY OF THIS PROSPECTUS SUPPLEMENT OR THE
      PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
 
                              -------------------
 
                  PRICE OF NOTES 99.369% AND ACCRUED INTEREST
                PRICE OF DEBENTURES 99.070% AND ACCRUED INTEREST
                              -------------------
 
<TABLE>
<CAPTION>
                                                                                  UNDERWRITING
                                                                                 DISCOUNTS AND        PROCEEDS TO
                                                           PRICE TO PUBLIC(1)    COMMISSIONS(2)      COMPANY(1)(3)
                                                           ------------------  ------------------  ------------------
<S>                                                        <C>                 <C>                 <C>
PER NOTE.................................................       99.369%              .650%              98.719%
    TOTAL................................................     $99,369,000           $650,000          $98,719,000
PER DEBENTURE............................................       99.070%              .875%              98.195%
    TOTAL................................................     $198,140,000         $1,750,000         $196,390,000
</TABLE>
 
- ---------
 
  (1) PLUS ACCRUED INTEREST FROM MAY 1, 1998.
 
  (2) THE COMPANY AND UNOCAL HAVE AGREED TO INDEMNIFY THE UNDERWRITERS AGAINST
      CERTAIN LIABILITIES, INCLUDING LIABILITIES UNDER THE SECURITIES ACT OF
      1933, AS AMENDED. SEE "UNDERWRITERS."
 
  (3) BEFORE DEDUCTING EXPENSES PAYABLE BY THE COMPANY ESTIMATED AT $200,000.
                            ------------------------
 
    THE OFFERED SECURITIES ARE OFFERED, SUBJECT TO PRIOR SALE, WHEN, AS AND IF
ACCEPTED BY THE UNDERWRITERS. IT IS EXPECTED THAT DELIVERY OF THE OFFERED
SECURITIES WILL BE MADE ON OR ABOUT MAY 6, 1998 THROUGH THE BOOK-ENTRY
FACILITIES OF THE DEPOSITARY AGAINST PAYMENT THEREFOR IN IMMEDIATELY AVAILABLE
FUNDS.
                              -------------------
 
MORGAN STANLEY DEAN WITTER                                  GOLDMAN, SACHS & CO.
          CREDIT SUISSE FIRST BOSTON
                     J.P. MORGAN & CO.
                              NATIONSBANC MONTGOMERY SECURITIES LLC
                                           SALOMON SMITH BARNEY
 
MAY 1, 1998
<PAGE>
    NO DEALER, SALESPERSON OR OTHER INDIVIDUAL HAS BEEN AUTHORIZED TO GIVE ANY
INFORMATION OR TO MAKE ANY REPRESENTATIONS OTHER THAN THOSE CONTAINED OR
INCORPORATED BY REFERENCE IN THIS PROSPECTUS SUPPLEMENT OR THE ACCOMPANYING
PROSPECTUS, AND IF GIVEN OR MADE, SUCH INFORMATION OR REPRESENTATIONS MUST NOT
BE RELIED UPON AS HAVING BEEN AUTHORIZED BY THE COMPANY, BY UNOCAL OR BY THE
UNDERWRITERS. NEITHER THIS PROSPECTUS SUPPLEMENT NOR THE ACCOMPANYING PROSPECTUS
CONSTITUTES AN OFFER TO SELL OR THE SOLICITATION OF AN OFFER TO BUY ANY
SECURITIES OTHER THAN THE SECURITIES DESCRIBED IN THIS PROSPECTUS SUPPLEMENT OR
AN OFFER TO SELL OR THE SOLICITATION OF AN OFFER TO BUY SUCH SECURITIES IN ANY
JURISDICTION TO ANY PERSONS TO WHOM IT IS UNLAWFUL TO MAKE SUCH OFFER IN SUCH
JURISDICTION. THE DELIVERY OF THIS PROSPECTUS SUPPLEMENT OR THE ACCOMPANYING
PROSPECTUS OR ANY SALE MADE HEREUNDER DOES NOT IMPLY THAT THERE HAS BEEN NO
CHANGE IN THE AFFAIRS OF THE COMPANY OR UNOCAL SINCE THE DATE HEREOF OR THAT THE
INFORMATION CONTAINED HEREIN OR THEREIN IS CORRECT AS OF ANY TIME SUBSEQUENT TO
THE DATE ON WHICH SUCH INFORMATION IS GIVEN.
 
                            ------------------------
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                                                                                PAGE
                                                                                                                -----
<S>                                                                                                          <C>
                                                 PROSPECTUS SUPPLEMENT
 
Description of the Offered Securities......................................................................         S-3
Recent Developments........................................................................................         S-6
Underwriters...............................................................................................         S-6
Legal Matters..............................................................................................         S-7
Experts....................................................................................................         S-7
 
                                                       PROSPECTUS
 
Available Information......................................................................................           2
Incorporation of Certain Documents by Reference............................................................           2
Use of Proceeds............................................................................................           3
The Company and Unocal.....................................................................................           3
Historical Condensed Consolidated Selected Financial Information...........................................           4
Description of the Debt Securities.........................................................................           5
Description of the Preferred Stock.........................................................................          18
Description of the Common Stock............................................................................          23
Description of the Warrants................................................................................          24
Limitations on the Issuance of Bearer Securities...........................................................          26
Plan of Distribution.......................................................................................          27
Experts....................................................................................................          28
Legal Matters..............................................................................................          28
</TABLE>
 
                            ------------------------
 
    CERTAIN PERSONS PARTICIPATING IN THIS OFFERING MAY ENGAGE IN TRANSACTIONS
THAT STABILIZE, MAINTAIN OR OTHERWISE AFFECT THE PRICES OF THE OFFERED
SECURITIES, SPECIFICALLY, THE UNDERWRITERS MAY BID FOR AND PURCHASE THE OFFERED
SECURITIES IN THE OPEN MARKET. FOR A DISCUSSION OF THESE ACTIVITIES, SEE
"UNDERWRITERS."
 
                                      S-2
<PAGE>
                     DESCRIPTION OF THE OFFERED SECURITIES
 
    THE FOLLOWING DESCRIPTION OF THE PARTICULAR TERMS OF THE OFFERED SECURITIES
(REFERRED TO IN THE ACCOMPANYING PROSPECTUS AS THE "OFFERED DEBT SECURITIES")
SUPPLEMENTS, AND TO THE EXTENT INCONSISTENT THEREWITH REPLACES, THE DESCRIPTION
THEREOF IN THE ACCOMPANYING PROSPECTUS, TO WHICH REFERENCE IS HEREBY MADE. THE
FOLLOWING SUMMARY OF THE OFFERED SECURITIES IS QUALIFIED IN ITS ENTIRETY BY
REFERENCE TO THE SENIOR INDENTURE REFERRED TO IN THE ACCOMPANYING PROSPECTUS.
CAPITALIZED TERMS NOT OTHERWISE DEFINED HEREIN OR IN THE ACCOMPANYING PROSPECTUS
HAVE THE MEANINGS GIVEN TO THEM IN THE SENIOR INDENTURE.
 
GENERAL
 
    The Notes and the Debentures offered hereby each constitutes a series of
Senior Debt Securities under the Senior Indenture described in the accompanying
Prospectus limited, with respect to the Notes, to $100 million aggregate
principal amount and, with respect to the Debentures, to $200 million aggregate
principal amount. The Notes and the Debentures will be unconditionally
guaranteed, as to the payment of principal, interest and premium, if any, by
Unocal. References to payments of principal in this Prospectus Supplement shall
include payments of premium, if any. Please refer to the accompanying Prospectus
for a detailed summary of additional provisions of the Notes, the Debentures and
the Senior Indenture under which the Notes and the Debentures will be issued.
The Notes will mature on May 1, 2008, and the Debentures will mature on May 1,
2028. The Notes and Debentures will bear interest at the rates shown in their
respective titles from May 1, 1998 or from the most recent Interest Payment Date
to which interest has been paid or provided for, payable semi-annually on May 1
and November 1 of each year, commencing November 1, 1998, to the persons in
whose names the Offered Securities are registered at the close of business on
the April 15 or October 15, as the case may be, next preceding such Interest
Payment Date. The Offered Securities are not entitled to the benefit of any
sinking fund. The Offered Securities will be sold in denominations of $1,000 and
integral multiples of $1,000 in excess thereof.
 
    The covenant provisions of the Senior Indenture described under the caption
"Description of Debt Securities--Certain Covenants of Unocal" in the
accompanying Prospectus will apply to the Offered Securities. The defeasance and
covenant defeasance provisions of the Senior Indenture described under the
caption "Description of Debt Securities--Defeasance" in the accompanying
Prospectus will apply to the Offered Securities.
 
REDEMPTION AT THE OPTION OF THE COMPANY
 
    The Offered Securities will be redeemable, in whole or in part, at the
option of the Company on any date (a "Redemption Date"), at a redemption price
equal to the greater of (a) 100% of the principal amount of the Offered
Securities to be redeemed or (b) the sum of the present values of the remaining
scheduled payments of principal and interest thereon (exclusive of interest
accrued to such Redemption Date) discounted to such Redemption Date on a
semi-annual basis (assuming a 360-day year consisting of twelve 30-day months)
at the Treasury Rate plus 10 basis points (in the case of the Notes) or 20 basis
points (in the case of the Debentures), plus, in either case, accrued and unpaid
interest on the principal amount being redeemed to such Redemption Date (the
"Redemption Price"); provided, however, that installments of interest on Offered
Securities that are due and payable on an Interest Payment Date falling on or
prior to the relevant Redemption Date shall be payable to the holders of such
Offered Securities registered as such at the close of business on the relevant
Regular Record Date according to their terms and the provisions of the Senior
Indenture.
 
    "Treasury Rate" means with respect to the Redemption Date for the Offered
Securities, (a) the yield, under the heading that represents the average for the
immediately preceding week, appearing in the most recently published statistical
release designated "H.15(519)" or any successor publication that is published
weekly by the Board of Governors of the Federal Reserve System and that
establishes yields on actively traded United States Treasury securities adjusted
to constant maturity under the caption "Treasury Constant Maturities," for the
maturity corresponding to the Comparable Treasury Issue (if no maturity is
within three months before or after the Maturity Date, yields for the two
published maturities most closely corresponding to the Comparable Treasury Issue
shall be determined and the Treasury Rate shall be
 
                                      S-3
<PAGE>
interpolated or extrapolated from such yields on a straight-line basis, rounding
to the nearest month) or (b) if such release (or any successor release) is not
published during the week preceding the calculation date or does not contain
such yields, the rate per annum equal to the semi-annual equivalent yield to
maturity of the Comparable Treasury Issue, calculated using a price for the
Comparable Treasury Issue (expressed as a percentage of its principal amount)
equal to the Comparable Treasury Price for such Redemption Date. The Treasury
Rate shall be calculated on the third Business Day preceding the Redemption
Date.
 
    "Comparable Treasury Issue" means the United States Treasury security
selected by the Independent Investment Banker as having a maturity comparable to
the remaining term of the Offered Securities to be redeemed that would be
utilized, at the time of selection and in accordance with customary financial
practice, in pricing new issues of corporate debt securities of comparable
maturity to the remaining term of the Offered Securities.
 
    "Independent Investment Banker" means Morgan Stanley & Co. Incorporated or,
if such firm is unwilling or unable to select the Comparable Treasury Issue, an
independent investment banking institution of national standing appointed by the
Senior Trustee after consultation with the Company.
 
    "Comparable Treasury Price" means, with respect to any Redemption Date, (a)
the average of four Reference Treasury Dealer Quotations for such Redemption
Date, after excluding the highest and lowest such Reference Treasury Dealer
Quotations, or (b) if the Senior Trustee obtains fewer than four such Reference
Treasury Dealer Quotations the average of all such Quotations.
 
    "Reference Treasury Dealer" means each of Morgan Stanley & Co. Incorporated
and Goldman, Sachs & Co. and their respective successors; provided, however,
that if either of the foregoing shall cease to be a primary U.S. Government
securities dealer in New York City (a "Primary Treasury Dealer"), the Company
will substitute therefore another Primary Treasury Dealer.
 
    "Reference Treasury Dealer Quotations" means, with respect to each Reference
Treasury Dealer and any Redemption Date, the average, as determined by the
Senior Trustee, of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) quoted in
writing to the Senior Trustee by such Reference Treasury Dealer at 5:00 p.m.,
New York City time, on the third Business Day preceding such Redemption Date.
 
    Notice of any redemption by the Company will be mailed at least 30 days but
not more than 60 days before any Redemption Date to each holder of Offered
Securities to be redeemed. If less than all the Notes or the Debentures are to
be redeemed at the option of the Company, the Senior Trustee shall select, in
such manner as it shall deem fair and appropriate, the Offered Securities of
such series to be redeemed in whole or in part.
 
    Unless the Company defaults in payment of the Redemption Price, on and after
any Redemption Date interest will cease to accrue on the Offered Securities or
portions thereof called for redemption.
 
BOOK-ENTRY SYSTEM
 
    Upon issuance, the Offered Securities will be represented by global
securities (the "Global Securities"). The Global Securities representing the
Offered Securities will be deposited with, or on behalf of, The Depository Trust
Company, New York, New York (the "Depositary" or "DTC"). Upon the issuance of
the Global Securities, the Depositary or its nominee will credit the accounts of
persons for which it holds with the respective principal or face amounts of the
Offered Securities represented by such Global Securities. Ownership of
beneficial interests in the Global Securities will be limited to persons that
have accounts with the Depositary ("participants") or persons that may hold
interests through participants. Ownership of beneficial interests by
participants in the Global Securities will be shown on, and the transfer of that
ownership will be effected only through, records maintained by the Depositary.
Ownership of beneficial interests in such Global Securities by persons that hold
through participants will be shown on, and the transfer of that ownership
interest within such participant will be effected only through, records
maintained by such participant. The laws of some jurisdictions require that
certain purchasers of securities
 
                                      S-4
<PAGE>
take physical delivery of such securities in definitive form. Such limits and
such laws may impair the ability to acquire or transfer beneficial interests in
the Global Securities.
 
    Payment of principal of and interest on the Offered Securities will be made
to the Depositary or its nominee, as the case may be, as the sole registered
owner and holder of the Global Securities for all purposes under the Senior
Indenture. Neither the Company, Unocal, the Senior Trustee nor any agent of the
Company, Unocal or Senior Trustee will have any responsibility or liability for
any aspect of the Depositary's records relating to or payments made on account
of beneficial ownership interests in the Global Securities or for maintaining,
supervising or reviewing any of the Depositary's records relating to such
beneficial ownership interests.
 
    The Company has been advised by the Depositary that upon receipt of any
payment of principal of or interest on the Global Securities, the Depositary
will immediately credit, on its book-entry registration and transfer system, the
accounts of participants with payments in amounts proportionate to their
respective beneficial interests in the principal or face amount of such Global
Securities as shown on the records of the Depositary. Payments by participants
to owners of beneficial interests in the Global Securities held through such
participants will be governed by standing instructions and customary practices
as is now the case with securities held for customer accounts registered in
"street name" and will be the sole responsibility of such participants.
 
    The Global Securities may not be transferred except as a whole by the
Depositary to a nominee of the Depositary. The Global Securities representing
the Offered Securities are exchangeable for certificated Offered Securities only
if (x) the Depositary notifies the Company that it is unwilling or unable to
continue as Depositary for such Global Securities or if at any time the
Depositary ceases to be a clearing agency registered under the Securities
Exchange Act of 1934, as amended (the "Exchange Act"), and the Company fails
within 90-days thereafter to appoint a successor or (y) the Company in its sole
discretion determines that such Global Securities shall be exchangeable. In such
event, the Company will issue Offered Securities in certificated form in
exchange for the Global Securities. In any such instance, an owner of a
beneficial interest in the Global Securities will be entitled to physical
delivery in certificated form of Offered Securities equal in principal amount to
such beneficial interest and to have such Offered Securities registered in its
name. Offered Securities so issued in certificated form will be issued in
denominations of $1,000 or any larger amount that is an integral multiple
thereof and will be issued in registered form only, without coupons. Subject to
the foregoing, the Global Securities are not exchangeable, except for Global
Securities of like denomination to be registered in the name of the Depositary
or its nominee.
 
    So long as the Depositary, or its nominee, is the registered owner of the
Global Securities, such Depositary or such nominee, as the case may be, will be
considered the sole owner or holder of the Offered Securities represented by
such Global Securities for the purposes of receiving payment on the Offered
Securities, receiving notices and for all other purposes under the Senior
Indenture and the Offered Securities. Beneficial interests in Offered Securities
will be evidenced only by, and transfer thereof will be effected only through,
records maintained by the Depositary and its participants. Except as provided
herein, owners of beneficial interests in the Global Securities will not be
entitled to and will not be considered the holders thereof for any purposes
under the Senior Indenture. Accordingly, each person owning a beneficial
interest in such Global Securities must rely on the procedures of the
Depositary, and, if such person is not a participant, on the procedures of the
participant through which such person owns its interest, to exercise any rights
of a Holder under the Senior Indenture. The Depositary will not consent or vote
with respect to the Global Securities representing the Offered Securities. Under
its usual procedures, the Depositary mails an Omnibus Proxy to the Company as
soon as possible after the applicable record date. The Omnibus Proxy assigns
Cede & Co.'s (the Depositary's partnership nominee) consenting or voting rights
to those participants to whose accounts the Offered Securities are credited on
the applicable record date (identified in a listing attached to the Omnibus
Proxy).
 
    The Depositary has advised the Company that the Depositary is a
limited-purpose trust company organized under the New York Banking Law, a
"banking organization" within the meaning of the New
 
                                      S-5
<PAGE>
York Banking Law, a member of the Federal Reserve System, a "clearing
corporation" within the meaning of the New York Uniform Commercial Code, and a
"clearing agency" registered pursuant to the provisions of Section 17A of the
Exchange Act. The Depositary was created to hold the securities of its
participants and to facilitate the clearance and settlement of securities
transactions among its participants through electronic book-entry changes in
accounts of the participants, thereby eliminating the need for physical movement
of securities certificates. The Depositary's participants include securities
brokers and dealers, banks, trust companies, clearing corporations, and certain
other organizations, some of whom (and/or their representatives) own the
Depositary. Access to the Depositary's book-entry system is also available to
others, such as banks, brokers, dealers and trust companies that clear through
or maintain a custodial relationship with a participant, either directly or
indirectly. The rules applicable to the Depositary and its participants are on
file with the Securities and Exchange Commission.
 
SENIOR TRUSTEE
 
    The Senior Trustee under the Senior Indenture is Chase Manhattan Bank and
Trust Company, National Association (successor by merger to Chemical Trust
Company of California). ChaseMellon Shareholder Services, L.L.C., an indirect
affiliate of the Senior Trustee (as successor in interest to Chemical Trust
Company of California), is the transfer agent and registrar for the Unocal
Common Stock and (as successor in interest to Chemical Bank) is the
administrator of the Unocal Dividend Reinvestment and Common Stock Purchase
Plan. The Chase Manhattan Bank (as successor in interest to Chemical Trust
Company of California), an indirect affiliate of the Senior Trustee, is the
Rights Agent under the Rights Agreement, dated as of January 29, 1990, referred
to in the accompanying Prospectus and is a lending bank under the Company's
$1,000,000,000 Credit and Guarantee Agreement, dated as of October 10, 1997. The
Company and Unocal maintain and may in the future maintain other banking
relationships with The Chase Manhattan Bank in the ordinary course of business.
See "Description of the Debt Securities-- The Trustees" in the accompanying
prospectus.
 
                              RECENT DEVELOPMENTS
 
    On April 30, 1998, Standard & Poor's revised its outlook for Unocal and its
subsidiaries (including the Company) to negative from stable. Standard & Poor's
has reaffirmed its ratings for Unocal and the Company.
 
                                  UNDERWRITERS
 
    Under the terms and subject to the conditions contained in an Underwriting
Agreement dated the date hereof, the Underwriters named below have severally
agreed to purchase, and the Company has agreed to sell to them, severally, the
respective principal amount of the Offered Securities set forth opposite their
respective names below:
 
<TABLE>
<CAPTION>
                                                                               PRINCIPAL AMOUNT  PRINCIPAL AMOUNT
                                    NAME                                           OF NOTES       OF DEBENTURES
- -----------------------------------------------------------------------------  ----------------  ----------------
<S>                                                                            <C>               <C>
Morgan Stanley & Co. Incorporated............................................   $   30,000,000    $   60,000,000
Goldman, Sachs & Co..........................................................       30,000,000        60,000,000
Credit Suisse First Boston Corporation.......................................       10,000,000        20,000,000
J.P. Morgan Securities Inc...................................................       10,000,000        20,000,000
NationsBanc Montgomery Securities LLC........................................       10,000,000        20,000,000
Salomon Brothers Inc.........................................................       10,000,000        20,000,000
                                                                               ----------------  ----------------
    Total....................................................................   $  100,000,000    $  200,000,000
                                                                               ----------------  ----------------
                                                                               ----------------  ----------------
</TABLE>
 
    The Underwriting Agreement provides that the obligation of the several
Underwriters to pay for and accept delivery of the Offered Securities is subject
to the approval of certain legal matters by their counsel and to certain other
conditions. The Underwriters are obligated to take and pay for all of the
Offered Securities if any are taken.
 
                                      S-6
<PAGE>
    The Underwriters initially propose to offer part of the Offered Securities
directly to the public at the public offering price set forth on the cover page
hereof and part to certain dealers at a price that represents a concession not
in excess of .40% of the principal amount in the case of the Notes and .50% of
the principal amount in the case of the Debentures. Any Underwriter may allow,
and any such dealers may reallow, a concession to certain other dealers not in
excess of .25% of the principal amount in the case of the Notes and .25% of the
principal amount in the case of the Debentures. After the initial offering of
the Offered Securities, the offering price and other selling terms may from time
to time be varied by the Underwriters.
 
    The Company and Unocal have agreed to indemnify the several Underwriters
against certain liabilities, including liabilities under the Securities Act of
1933, as amended.
 
    The Offered Securities are new issues of securities with no established
trading market. The Company does not intend to apply for listing of any of the
Offered Securities on a national securities exchange, but has been advised by
the Underwriters that they presently intend to make a market in the Offered
Securities, as permitted by applicable laws and regulations. The Underwriters
are not obligated, however, to make a market in the Offered Securities and any
such market-making may be discontinued at any time at the sole discretion of the
Underwriters. Accordingly, no assurance can be given as to the liquidity of, or
trading markets for, the Offered Securities.
 
    In order to facilitate the offering of the Offered Securities, the
Underwriters may engage in transactions that stabilize, maintain or otherwise
affect the price of the Offered Securities. In addition, to stabilize the price
of the Offered Securities, the Underwriters may bid for and purchase the Offered
Securities in the open market. Finally, the Underwriters may reclaim selling
concessions allowed to an Underwriter or a dealer for distributing the Offered
Securities in the offering, if the Underwriter repurchases previously
distributed Offered Securities in transactions to cover Underwriter short
positions, in stabilization transactions or otherwise. Any of these activities
may stabilize or maintain the market price for the Offered Securities above
independent market levels. The Underwriters are not required to engage in these
activities and may end any of these activities at any time.
 
    Settlement for the Offered Securities will be made in immediately available
funds and all secondary trading in the Offered Securities will settle in
immediately available funds.
 
    From time to time, the Underwriters or their affiliates engage in
transactions with and perform services for the Company and its affiliates in the
ordinary course of business.
 
                                 LEGAL MATTERS
 
    Legal matters in connection with the issuance and sale of the Offered
Securities will be passed upon for the Company and Unocal by Dennis P.R. Codon,
Esq., Vice President, General Counsel and Chief Legal Officer of the Company and
Unocal, and for the Underwriters by Gibson, Dunn & Crutcher LLC, San Francisco,
California. As of April 30, 1998, Mr. Codon owned beneficially 23,114 shares of
Unocal Common Stock, which included 10,470 restricted shares that vest in 1999
through 2003. He also held options to purchase 57,088 shares of Unocal Common
Stock at prices ranging from $26.375 to $38.8125, with expiration dates ranging
from 2003 to 2008. In addition, Mr. Codon held 15,800 performance shares units
awarded to him in 1995 through 1998.
 
                                    EXPERTS
 
    The consolidated financial statements and financial statement schedule of
Unocal and its subsidiaries as of December 31, 1997 and 1996, and for each of
the three years in the period ended December 31, 1997, included in Unocal's
Annual Report on Form 10-K for the year ended December 31, 1997, and
incorporated by reference in the accompanying Prospectus, have been incorporated
therein in reliance on the report of Coopers & Lybrand L.L.P., independent
accountants, which report is incorporated by reference therein, and on the
authority of that firm as experts in accounting and auditing. Such report
includes an explanatory paragraph with respect to a change in the method of
accounting for the impairment of long-lived assets and long-lived assets to be
disposed of in 1995.
 
                                      S-7
<PAGE>
                                     [LOGO]
 
                                 $1,188,800,000
 
                        UNION OIL COMPANY OF CALIFORNIA
 
            DEBT SECURITIES AND WARRANTS TO PURCHASE DEBT SECURITIES
     WITH PAYMENT OF PRINCIPAL, INTEREST AND PREMIUM, IF ANY, GUARANTEED BY
                               UNOCAL CORPORATION
                            ------------------------
 
                               UNOCAL CORPORATION
 
             COMMON STOCK, PREFERRED STOCK AND WARRANTS TO PURCHASE
                        COMMON STOCK AND PREFERRED STOCK
                            ------------------------
 
    Union Oil Company of California (the "Company") intends to offer from time
to time in one or more series debt securities consisting of unsecured
debentures, notes or other evidences of indebtedness (the "Debt Securities"). At
the option of the Company, the Debt Securities may be offered as Senior Debt
Securities ("Senior Debt Securities") and as Subordinated Debt Securities
("Subordinated Debt Securities"). Unocal Corporation ("Unocal"), the parent
company of the Company, will guarantee the payment of principal, interest and
premium, if any, on the Debt Securities. The Company and Unocal may also offer
from time to time warrants to purchase guaranteed Debt Securities ("Debt
Warrants"), which may be issued independently or together with guaranteed Debt
Securities. Unocal may offer from time to time Common Stock ("Unocal Common
Stock"), Preferred Stock ("Unocal Preferred Stock") and warrants to purchase
Unocal Common or Preferred Stock ("Equity Warrants"), which may be issued
independently or together with Unocal Common Stock or Unocal Preferred Stock.
Such Unocal Common Stock and Unocal Preferred Stock may also be issued upon
conversion or exchange of Debt Securities and such Unocal Common Stock may be
issued upon conversion of Unocal Preferred Stock. The Debt Securities, Debt
Warrants, Unocal Common Stock, Unocal Preferred Stock and Equity Warrants are
referred to collectively as the "Securities." No more than an aggregate of
$1,888,800,000 public offering price of Securities, including the exercise price
of Debt Warrants and Equity Warrants, may be sold pursuant to this Prospectus.
The Securities may be sold for United States dollars, foreign currency or
currency units.
 
    Certain specific terms of the particular Securities in respect of which this
Prospectus is being delivered will be set forth in the accompanying Prospectus
Supplement, including, where applicable, (i) in the case of Debt Securities, the
title, aggregate principal amount, authorized denominations, maturity, interest
rate (which may be fixed or variable) and time of payment of interest, terms for
redemption, terms for sinking fund payments, terms for conversion or exchange
into other Securities, currency or currencies of denomination and payment (if
other than U.S. dollars), listing on a securities exchange and any other terms
in connection with the offering and sale of the Debt Securities in respect of
which this Prospectus is delivered, as well as the initial public offering
price; (ii) in the case of Unocal Preferred Stock, the specific title, number of
shares, dividend (including the method of calculation), seniority, liquidation,
redemption, voting and other rights, terms for any conversion or exchange into
other Securities, listing on a securities exchange, initial public offering
price and any other terms; (iii) in the case of Unocal Common Stock, the number
of shares and the terms of the offering thereof; and (iv) in the case of Debt
Warrants and Equity Warrants, the designation and number, exercise price, any
listing of the Debt Warrants, Equity Warrants or the underlying Securities on a
securities exchange and any other terms in connection with the offering, sale
and exercise of the Debt Warrants and Equity Warrants.
 
    Debt Securities of a series may be issued in registered form, in a form
registered as to principal only, or in bearer form (with or without coupons
attached), or any combination of such forms. In addition, all or a portion of
the Debt Securities may be issued in temporary or definitive global form. Debt
Securities in bearer form are offered only outside the United States to
non-United States persons and to offices located outside the United States of
certain United States financial institutions and other exempt persons. See
"Limitations on the Issuance of Bearer Securities."
 
THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE SECURITIES
 AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION PASSED UPON THE
   ACCURACY OR ADEQUACY  OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                        CONTRARY IS A CRIMINAL OFFENSE.
                           --------------------------
 
    The Securities will be sold directly, through agents designated from time to
time or through underwriters or dealers, which may be a group of underwriters.
The Securities may also be exchanged for outstanding securities of the Company
or Unocal or both and resold by the holder pursuant to this Prospectus in the
over-the-counter market, on the New York Stock Exchange, through negotiated
transactions or otherwise, at market prices prevailing at the time of sale or at
prices otherwise negotiated. The terms of any such exchange and the method of
resale by the holder will be set forth in a Prospectus Supplement. If any agents
of the Company or Unocal or any dealers or underwriters are involved in the sale
of the Securities, the names of such agents, underwriters or dealers and any
applicable commissions or discounts will be set forth in a Prospectus
Supplement.
 
       THIS PROSPECTUS MAY NOT BE USED TO CONSUMMATE SALES OF SECURITIES
                 UNLESS ACCOMPANIED BY A PROSPECTUS SUPPLEMENT.
 
                THE DATE OF THIS PROSPECTUS IS FEBRUARY 3, 1995.
<PAGE>
                             AVAILABLE INFORMATION
 
    Unocal is subject to the informational requirements of the Securities
Exchange Act of 1934 (the "Exchange Act") and in accordance therewith files
reports, proxy statements and other information with the Securities and Exchange
Commission (the "Commission"). Such reports, proxy statements and other
information filed by Unocal may be inspected and copied at the public reference
facilities maintained by the Commission at 450 Fifth Street, N.W., Washington,
D.C. 20549, and at the following Regional Offices of the Commission: 7 World
Trade Center, 13th Floor, New York, New York 10048; and 500 West Madison Street,
Suite 1400, Chicago, Illinois 60661-2511. Copies of such material may also be
obtained by mail from the Public Reference Section of the Commission at 450
Fifth Street, N.W., Washington, D.C. 20549, at prescribed rates. In addition,
such reports, proxy statements and other information concerning Unocal may be
inspected at the offices of the New York Stock Exchange, 20 Broad Street, 17th
Floor, New York, New York 10005, the Chicago Stock Exchange, 440 South LaSalle
Street, Suite 518, Chicago, Illinois 60605-1070 and the Pacific Stock Exchange,
115 Sansome Street, 3rd Floor, San Francisco, California 94104.
 
    Prior to the date of this Prospectus, the Company filed reports and other
information with the Commission in accordance with the reporting requirements of
the Exchange Act. Such reports and other information filed by the Company may be
inspected and copied at, and obtained by mail from, the offices of the
Commission listed above. The Company has received advice from the Division of
Corporation Finance of the Commission that it will not object if the Company
ceases to file such reports, since debt securities of the Company are fully and
unconditionally guaranteed as to payment by Unocal. Accordingly, the Company has
ceased to file such reports with the Commission. Notes to the consolidated
financial statements of Unocal included in Unocal reports filed hereafter will
contain summarized financial information regarding the Company.
 
    Unocal and the Company have filed with the Commission registration
statements on Form S-3 (together with all amendments and exhibits thereto, the
"Registration Statements") under the Securities Act of 1933. This Prospectus and
the accompanying Prospectus Supplement do not contain all of the information set
forth in the Registration Statements, certain parts of which are omitted in
accordance with the rules and regulations of the Commission. For further
information, reference is made to the Registration Statements, which may be
examined without charge at the public reference facilities maintained by the
Commission at 450 Fifth Street, N.W., Washington, D.C. 20549. Copies thereof may
be obtained from the Commission upon payment of the prescribed fees.
 
                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE
 
    The following documents filed with the Commission are incorporated into this
Prospectus by reference: (i) the Annual Report on Form 10-K for the fiscal year
ended December 31, 1993 of Unocal, as amended by Amendments Nos. 1 and 2 on Form
10-K/A; (ii) the Annual Report on Form 10-K for the fiscal year ended December
31, 1993 of the Company, as amended by Amendment No. 1 on Form 10-K/A; (iii) the
Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 1994,
June 30, 1994 and September 30, 1994, the last as amended by Amendment No. 1 on
Form 10-Q/A, of Unocal and the Company; (iv) the Current Reports on Form 8-K
dated January 12, 1994, January 31, 1994, March 2, 1994, March 24, 1994, April
25, 1994, June 27, 1994, June 28, 1994, July 22, 1994, July 25, 1994, August 25,
1994, September 9, 1994, September 30, 1994, October 31, 1994, November 2, 1994,
December 5, 1994, January 30, 1995 and January 31, 1995 of Unocal; and (v) the
Current Reports on Form 8-K dated March 2, 1994, June 27, 1994, June 28, 1994,
August 25, 1994, September 30, 1994 and December 5, 1994 of the Company. All
documents filed by Unocal pursuant to Section 13(a), 13(c), 14 or 15(d) of the
Exchange Act subsequent to the date of this Prospectus and prior to the
termination of the offering of the Securities shall be deemed to be incorporated
by reference in this Prospectus and to be a part hereof from the date of filing
of such documents. Any statement contained herein or in a document all or a
portion of which is incorporated or deemed to be incorporated by reference
herein shall be deemed to be modified or superseded for purposes of this
Prospectus to the extent that a statement contained herein or in any other
 
                                       2
<PAGE>
subsequently filed document which also is or is deemed to be incorporated by
reference herein modifies or supersedes such statement. Any such statement so
modified or superseded shall not be deemed, except as so modified or superseded,
to constitute a part of this Prospectus.
 
    Unocal and the Company will provide without charge to each person to whom a
copy of this Prospectus is delivered, upon the written or oral request of any
such person, a copy of any or all of the documents incorporated herein by
reference (not including the exhibits to such documents, unless such exhibits
are specifically incorporated by reference in such documents). Requests for such
copies should be directed to: Unocal Corporation, 1201 West Fifth Street, Los
Angeles, California 90017, Attention: Corporate Secretary, telephone (213)
977-7600.
 
                                USE OF PROCEEDS
 
    The proceeds received by the Company from the sale of the Debt Securities
and Debt Warrants offered hereby, will be used by the Company and its affiliates
for general corporate purposes. The proceeds received by Unocal from the sale of
the Unocal Preferred Stock, Unocal Common Stock and Equity Warrants offered
hereby will be used for general corporate purposes, which are expected to
include contributions or loans to the Company. The gross proceeds anticipated to
be received by the Company and/or Unocal are estimated to be $1,188,800,000.
 
                             THE COMPANY AND UNOCAL
 
    The Company is principally engaged in the exploration for, and the
production, transportation and sale of, crude oil and natural gas in the United
States and foreign countries; and the manufacture, purchase, transportation and
marketing of petroleum and selected chemical products. The Company is also
engaged in the exploration for, and the production and sale of, geothermal
resources. Other operations include the production and marketing of specialty
minerals, and real estate development and sales.
 
    The Company was incorporated in California in 1890 and in 1983 became a
wholly owned operating subsidiary of Unocal. As of December 31, 1994, the net
assets of the Company represented approximately 100% of the net assets of
Unocal, based on book value. The Company is a California corporation and Unocal
is a Delaware corporation, each with its principal executive office at 1201 West
Fifth Street, Los Angeles, California 90017, telephone (213) 977-7600.
 
                                       3
<PAGE>
                       HISTORICAL CONDENSED CONSOLIDATED
                         SELECTED FINANCIAL INFORMATION
 
    The following historical condensed consolidated financial information of
Unocal and its subsidiaries, including the Company, for the five years ended
December 31, 1994, has been derived from and is qualified in its entirety by the
detailed financial statements and other financial information included in the
documents incorporated by reference herein. See "Incorporation of Certain
Documents by Reference."
 
                    SELECTED FINANCIAL INFORMATION OF UNOCAL
                (MILLIONS OF DOLLARS, EXCEPT PER SHARE AMOUNTS)
<TABLE>
<CAPTION>
                                                                YEAR ENDED DECEMBER 31,
                                                -------------------------------------------------------
                                                   1994        1993       1992       1991       1990
                                                -----------  ---------  ---------  ---------  ---------
                                                (UNAUDITED)
<S>                                             <C>          <C>        <C>        <C>        <C>
INCOME STATEMENT DATA
  Revenues....................................   $   7,965   $   8,344  $  10,061  $  10,895  $  11,808
  Earnings before cumulative effect of
    accounting changes (1)....................         124         343        196         73        401
    Per common share..........................         .36        1.27        .75        .31       1.71
  Net earnings (loss).........................        (153)        213        220         73        401
    Per common share..........................        (.78)        .73        .85        .31       1.71
 
<CAPTION>
                                                                    AT DECEMBER 31,
                                                -------------------------------------------------------
                                                   1994        1993       1992       1991       1990
                                                -----------  ---------  ---------  ---------  ---------
                                                (UNAUDITED)
<S>                                             <C>          <C>        <C>        <C>        <C>
BALANCE SHEET DATA
  Total assets (2)............................   $   9,337   $   9,706  $   9,892  $  10,345  $  10,157
  Long-term debt..............................       3,452       3,455      3,530      4,543      4,025
  Stockholders' equity........................       2,815       3,129      3,131      2,464      2,550
</TABLE>
 
- ------------------------
 
(1) The cumulative effect of accounting changes, which have been excluded,
    consisted of a charge of $277 million ($1.14 per common share) in 1994, a
    charge of $130 million ($.54 per common share) in 1993 and a credit of $24
    million ($.10 per common share) in 1992.
 
(2) Total assets prior to 1994 have been restated to reflect the
    reclassification of the accumulated allowance for future abandonment and
    restoration costs from the property account to a liability account.
 
                                       4
<PAGE>
                        RATIOS OF UNOCAL AND THE COMPANY
                                  (UNAUDITED)
<TABLE>
<CAPTION>
                                                                                              YEAR ENDED DECEMBER 31,
                                                                                 --------------------------------------------------
                                                                                    1994         1993         1992         1991
                                                                                    -----        -----        -----        -----
<S>                                                                              <C>          <C>          <C>          <C>
Ratio of Earnings to Fixed Charges(1)
  Unocal.......................................................................         1.7          2.5          1.7          1.4
  Company......................................................................         1.7          2.5          1.7          1.4
Ratio of Earnings to Combined Fixed Charges and Preferred Stock Dividends(1)(2)
  Unocal.......................................................................         1.5          2.2          1.6          1.4
 
<CAPTION>
 
                                                                                    1990
                                                                                    -----
<S>                                                                              <C>
Ratio of Earnings to Fixed Charges(1)
  Unocal.......................................................................         2.0
  Company......................................................................         2.2
Ratio of Earnings to Combined Fixed Charges and Preferred Stock Dividends(1)(2)
  Unocal.......................................................................         2.0
</TABLE>
 
- ------------------------
 
(1) For purposes of this ratio, earnings consist of earnings before the
    cumulative effect of accounting changes, before taxes on income and fixed
    charges. Fixed charges consist of interest on indebtedness (including
    capitalized interest) and capital lease obligations, amortization of debt
    discount, debt premium and issuance expense and that portion of operating
    lease rental expense which is representative of the interest factor (assumed
    to be one-third).
 
(2) For purposes of this ratio, preferred stock dividends are adjusted to a
    pre-tax basis.
 
                       DESCRIPTION OF THE DEBT SECURITIES
 
    Described below are certain general terms and provisions of the Debt
Securities to which a Prospectus Supplement may relate or for which Debt
Warrants may be exercisable. The particular terms of the Debt Securities and the
extent, if any, to which such general provisions may apply to a particular
series of Debt Securities ("Offered Debt Securities") will be described in the
Prospectus Supplement relating to such Offered Debt Securities.
 
    The Senior Debt Securities will be issued under an Indenture dated as of
February 3, 1995 (the "Senior Indenture") among the Company, Unocal and Chemical
Trust Company of California, as trustee (the "Senior Trustee"). The Subordinated
Debt Securities will be issued under a proposed indenture (the "Subordinated
Indenture") among the Company, Unocal and a trustee to be named in any
Prospectus Supplement relating to Subordinated Debt Securities (the
"Subordinated Trustee"). The Senior Indenture and the Subordinated Indenture are
referred to collectively as the "Indentures" and individually as an "Indenture."
The Senior Indenture incorporates, and the Subordinated Indenture will
incorporate, the Standard Multiple-Series Indenture Provisions, January 1991,
dated as of January 2, 1991 (the "Standard Provisions"), of the Company and
Unocal, which is filed as an exhibit to the Registration Statement. Neither of
the Indentures will limit the amount of Debt Securities which may be issued
thereunder (Section 2.01). Each of the Indentures will provide that Debt
Securities of any series may be issued thereunder up to the aggregate principal
amount which may be authorized from time to time by the Company.
 
    The following summaries of certain provisions of the Debt Securities and the
Indentures do not purport to be complete and are subject to, and qualified in
their entirety by reference to, all provisions of the Indentures, including the
definitions of certain terms used therein. Wherever particular sections of the
Indentures or terms that are defined in the Indentures are referred to herein or
in an accompanying Prospectus Supplement, it is intended that such sections or
terms will be incorporated by reference as a part of the statements made herein
or therein, and the statements are qualified in their entirety by such
reference. Unless otherwise indicated, references in this Prospectus or in an
accompanying Prospectus Supplement to particular sections of the Indentures are
to the Standard Provisions. Unless otherwise indicated, when used in this
Prospectus the term "principal" will mean principal of, and any premium on, the
Debt Securities.
 
                                       5
<PAGE>
GENERAL
 
    The Debt Securities will be direct, unsecured obligations of the Company and
will be fully and unconditionally guaranteed as to payment by Unocal. The Senior
Debt Securities and the related Guarantees will rank on a parity with all other
unsecured and unsubordinated indebtedness of the Company and Unocal,
respectively, and will have a right of payment prior to any Subordinated Debt
Securities, in the case of Senior Debt Securities, and prior to the Guarantees
of Subordinated Debt Securities, in the case of the Guarantees of the Senior
Debt Securities. The indebtedness represented by the Subordinated Debt
Securities and the Guarantees of the Subordinated Debt Securities will be
subordinated in right of payment to the prior payment in full of the Senior Debt
of the Company and Unocal, respectively, as described below under
"Subordination." The Debt Securities may be issued in one or more series with
the same or various maturities at or above par or with an original issue
discount. Offered Debt Securities bearing no interest or interest at a rate
which at the time of issuance is below market rates ("Original Issue Discount
Securities") will be sold at a discount (which may be substantial) below their
stated principal amount. In the event of redemption or acceleration of the
maturity of an Original Issue Discount Security, the amount payable to the
holder of such Security upon such redemption or acceleration will be determined
in accordance with the terms of the Security, but will be an amount less than
the amount payable at the Stated Maturity of such Security.
 
    Reference is made to the Prospectus Supplement relating to the Offered Debt
Securities for the following terms thereof:
 
        (1) the title of the Offered Debt Securities;
 
        (2) any limit upon the aggregate principal amount of the Offered Debt
    Securities;
 
        (3) the percentage of their principal amount for which the Offered Debt
    Securities will be issued;
 
        (4) the date or dates on which the principal of the Offered Debt
    Securities will be payable;
 
        (5) the rate or rates (which may be fixed or variable) at which the
    Offered Debt Securities will bear interest, if any, or the method by which
    such rate or rates will be determined;
 
        (6) the date or dates from which any such interest will accrue or the
    method by which such date or dates will be determined;
 
        (7) the dates on which payment of any such interest will be payable and
    the record dates for such interest payment dates;
 
        (8) the place or places where the principal of and any interest on the
    Offered Debt Securities (and Coupons, if any) will be payable and the
    offices or agencies of the Company maintained for such purposes and each
    office or agency where the Offered Debt Securities may be presented for
    registration of transfer or exchange;
 
        (9) the period or periods within which, the price or prices at which,
    and the terms and conditions upon which, the Offered Debt Securities may be
    redeemed in whole or in part, at the option of the Company;
 
       (10) the obligation of the Company, if any, to redeem, repay or purchase,
    the Offered Debt Securities pursuant to any sinking fund or analogous
    provision or at the option of a holder of an Offered Debt Security and the
    period or periods within which, the price or prices at which, and the terms
    and conditions upon which, the Offered Debt Securities will be redeemed,
    repaid or purchased, in whole or in part, pursuant to such obligation;
 
       (11) any additional restrictive covenants included for the benefit of
    holders of the Offered Debt Securities;
 
                                       6
<PAGE>
       (12) any additional Events of Default with respect to the Offered Debt
    Securities;
 
       (13) the principal amount of the Offered Debt Securities that are
    Original Issue Discount Securities payable upon declaration of acceleration
    of the maturity of the Offered Debt Securities;
 
       (14) the currency or currency unit for which the Offered Debt Securities
    may be purchased, the currency or currency unit in which the payment of
    principal and interest on such Offered Debt Securities will be payable, the
    right of the Company or the holder to elect a currency different from that
    in which the Offered Debt Securities are denominated for payments of
    principal and interest, and the Exchange Rate Agent, if any;
 
       (15) any index used to determine the amount of payments of principal of
    and interest on the Offered Debt Securities;
 
       (16) whether the Offered Debt Securities will be issued in registered
    form, in a form registered only as to principal, or in bearer form, or any
    combination thereof;
 
       (17) whether and on what terms the Offered Debt Securities will be
    convertible or exchangeable into shares of Unocal Preferred Stock or Unocal
    Common Stock;
 
       (18) whether any of the Offered Debt Securities will be issuable
    initially as a temporary Global Security (as defined in "Form, Exchange,
    Registration and Transfer") and whether any of the Offered Debt Securities
    are to be issuable as a permanent Global Security, or any combination
    thereof and, if so, the Depositary (as defined in "Global Securities") or
    Depositaries therefor;
 
       (19) if a temporary Global Security is to be issued with respect to such
    series, the requirements for certification of ownership by non-United States
    persons that will apply prior to (a) the issuance of a definitive Bearer
    Security (as defined in "Form, Exchange, Registration and Transfer") or (b)
    the payment of interest on an Interest Payment Date that occurs before the
    issuance of a definitive Bearer Security;
 
       (20) the circumstances under which Offered Debt Securities may be
    exchanged for Debt Securities issued in a different form;
 
       (21) any paying agents, transfer agents, registrars or other agents with
    respect to the Offered Debt Securities;
 
       (22) whether and under what circumstances the Company will pay additional
    amounts to any holder of Offered Debt Securities who is not a United States
    person (as defined under "Limitations on the Issuance of Bearer Securities")
    in respect of any tax, assessment or governmental charge required to be
    withheld or deducted and, if so, whether the Company will have the option to
    redeem rather than pay any additional amounts;
 
       (23) whether any of the provisions described in "Certain Covenants of
    Unocal," "Events of Default," "Subordination," "Conversion and Exchange,"
    "Form, Exchange, Registration and Transfer," and "Defeasance" will not apply
    to the Offered Debt Securities;
 
       (24) any other terms of the Offered Debt Securities not inconsistent with
    the applicable Indenture; and
 
       (25) a discussion of certain Federal income tax considerations, if
    required.
 
INTEREST AND FOREIGN CURRENCY
 
    Principal and interest will be payable, and the Offered Debt Securities will
be transferable, in the manner described in the Prospectus Supplement relating
to such Offered Debt Securities.
 
                                       7
<PAGE>
    If any of the Offered Debt Securities are sold for any foreign currency or
currency unit or if principal of or any interest on any of the Offered Debt
Securities is payable in any foreign currency or currency unit, the
restrictions, elections, tax consequences, specific terms and other information
with respect to such Offered Debt Securities and such foreign currency or
currency unit will be specified in a Prospectus Supplement.
 
GUARANTEES
 
    Under the terms of the Indentures and subject to the provisions thereof,
Unocal will fully and unconditionally guarantee to the holders from time to time
of the Debt Securities: (i) the full and prompt payment of the principal of any
Debt Securities and Coupons, if any, when and as the same become payable,
whether at the Stated Maturity thereof, by acceleration, call for redemption or
otherwise, and (ii) the full and prompt payment of any interest on any Debt
Securities and Coupons, if any, when and as the same becomes payable. The
Guarantees will remain in effect until the entire principal of and interest on
the Debt Securities has been paid in full or otherwise discharged in accordance
with the provisions of the Indentures (Section 5.01). In the event of a default
in the payment of principal of any Debt Security when and as the same becomes
payable, whether at the Stated Maturity thereof, by acceleration, call for
redemption or otherwise, or in the event of a default in any sinking fund
payment, or in the event of a default in the payment of any interest on any Debt
Security when and as the same becomes payable, the Trustee has the right to
proceed directly against Unocal without first proceeding against the Company or
exhausting any other remedies which the Trustee may have (Section 5.02). Any
right of payment of the holders of Senior Debt Securities under the related
Guarantees will be prior to the right of payment of the holders of Subordinated
Debt Securities under the related Guarantees.
 
CERTAIN COVENANTS OF UNOCAL
 
    LIMITATIONS ON LIENS.  The Senior Indenture provides that neither Unocal nor
any Restricted Subsidiary will issue, assume or guarantee any indebtedness for
money borrowed ("Debt") that is secured by a Mortgage upon (i) any domestic oil
or gas property of Unocal or a Restricted Subsidiary, (ii) any principal
domestic refining or manufacturing plant of Unocal or a Restricted Subsidiary,
or (iii) shares of stock or indebtedness of any Restricted Subsidiary, unless
the Senior Debt Securities will be secured equally and ratably with or prior to
such Debt. This covenant will not apply to (a) Mortgages on property or
securities of a corporation when it becomes a Restricted Subsidiary, (b)
purchase money Mortgages, (c) Mortgages existing at the time of acquisition of
property pursuant to a merger, consolidation or purchase of substantially all
the assets of the Seller, (d) any Mortgage securing Debt owing by a Restricted
Subsidiary to Unocal or to another Restricted Subsidiary, (e) Mortgages on
particular property incurred in connection with the exploration, drilling,
development, repair, alteration or improvement of such property, (f) Mortgages
on current assets or other personal property to secure Debt maturing in not more
than one year, or extensions, renewals or replacements of Mortgages referred to
in (a) through (e). Notwithstanding the foregoing, Unocal or one or more
Restricted Subsidiaries may issue, assume or guarantee Debt secured by a
Mortgage which would otherwise be subject to the foregoing restrictions if the
aggregate amount of such Debt, together with the aggregate principal amount of
all other such Debt of Unocal and its Restricted Subsidiaries then outstanding,
does not at such time exceed 20% of the Consolidated Net Assets of Unocal
(Senior Indenture Section 5.04).
 
    The following types of transactions, among others, will not be deemed to
create Debt secured by a Mortgage: (a) the sale or transfer of oil, oil shale,
gas or other minerals in place for a period of time until, or in an amount such
that, the transferee will realize therefrom a specified amount of money (however
determined) or a specified amount of such minerals or the sale or transfer of
any other interest in property of the character commonly referred to as a
"production payment" and (b) the placing of any Mortgage in favor of domestic or
foreign governmental bodies or agencies to secure payment, or the performance of
any other obligations, pursuant to any contract or statute or to secure any
indebtedness incurred for the
 
                                       8
<PAGE>
purpose of financing or refinancing all or a part of the purchase price or the
cost of construction of the property subject to such Mortgage (Senior Indenture
Section 5.04).
 
    The term "Mortgage" is defined as any mortgage, pledge, lien, security
interest, conditional sale or other title retention agreement or other similar
encumbrance (Senior Indenture Section 1.01).
 
    The term "oil or gas property" is defined as any interest owned by Unocal or
a Restricted Subsidiary in land which in the opinion of Unocal's Board of
Directors is capable of producing crude oil, natural gas or other hydrocarbons
in paying quantities and any interest in such substances produced or to be
produced (or the proceeds thereof) from said lands, but not including
exploration or production facilities or other improvements on said lands (Senior
Indenture Section 5.04).
 
    The term "Consolidated Net Assets" is defined as the total amount of assets
(less applicable reserves and other properly deductible items) of Unocal and its
consolidated Subsidiaries after deducting therefrom all liabilities and
liability items except Long-Term Debt, stockholders' equity and deferred income
taxes, which under generally accepted accounting principles would be included on
such consolidated balance sheet (Senior Indenture Section 1.01).
 
    The term "Restricted Subsidiary" is defined as the Company and any other
"Subsidiary" (i) substantially all of the assets and operations of which are
located within any one or more of the States of the United States and (ii) which
has assets in excess of 2% of the total consolidated assets of Unocal and its
consolidated Subsidiaries. The term "Subsidiary" is defined as any corporation,
association, or other business entity of which Unocal, either directly or
indirectly, has either (i) the voting power to elect a majority of the directors
of such corporation or (ii) other ownership interest representing more than 50%
ownership of such entity (Senior Indenture Section 1.01).
 
    LIMITATIONS ON SALE AND LEASEBACK.  Unocal will not, nor will it permit any
Restricted Subsidiary to, enter into any sale and leaseback arrangement (where
the lease runs for a term of more than five years) involving any domestic real
property, unless (i) Unocal or such Restricted Subsidiary is not restricted by
the above provisions from incurring Debt secured by a Mortgage on such property
or (ii) Unocal will apply within 90 days an amount equal to the greater of (a)
the fair value (as determined by the Board of Directors of Unocal) of such
property or (b) the proceeds of the sale of such property, to the retirement
(other than any mandatory retirement) of Long-Term Debt of Unocal or a
Restricted Subsidiary (other than Debt owned by Unocal or a Restricted
Subsidiary and Debt subordinated to the Senior Debt Securities) (Senior
Indenture Section 5.05). The foregoing limitations will not apply to any sale
and leaseback between Unocal and any of its Restricted Subsidiaries or between
any of its Restricted Subsidiaries.
 
    RESTRICTIONS ON MERGER AND SALE OF ASSETS.  Neither the Company nor Unocal
may consolidate with or merge into any other corporation, or transfer its
properties as an entirety or substantially as an entirety to any person, unless
(i) the person (if other than the Company or Unocal) formed by or resulting from
any such consolidation or merger or which has received the transfer of such
property and assets will be a corporation organized under the laws of the United
States or any state or territory thereof or the District of Columbia and will
assume payment of the principal of, and interest on, the Debt Securities and the
performance and observance of the Indentures and (ii) immediately after the
consolidation, merger, sale or conveyance, the surviving corporation or the
corporation to which the sale or conveyance was made will not be in default
under either Indenture (Section 12.01).
 
EVENTS OF DEFAULT
 
    The Senior Indenture defines, and the Subordinated Indenture will define, an
Event of Default with respect to any series of Debt Securities as being any one
of the following events: (i) default in the payment of any interest on any Debt
Security of that series when due, continued for 30 days after written notice has
been given by the Trustee to the Company or Unocal or by a holder to the Company
and the Trustee,
 
                                       9
<PAGE>
(ii) default in the payment of the principal of a Debt Security of that series
when due, (iii) default in the deposit of any sinking fund payment when and as
due by the terms of a Debt Security of such series, continued for 30 days after
written notice has been given by the Trustee to the Company or Unocal or by a
holder to the Company and the Trustee, (iv) default in any material respect in
the performance in any other of the Company's or Unocal's material covenants in
the applicable Indenture (other than a covenant included in such Indenture
solely for the benefit of another series of Debt Securities), continued for 90
days after written notice has been given by the Trustee to the Company or Unocal
or by holders of at least 25% in principal amount of the Outstanding Debt
Securities of such series to the Company and the Trustee, (v) a default
resulting in acceleration of any other indebtedness for borrowed money, in an
aggregate principal amount exceeding $50,000,000, of the Company or Unocal under
the terms of the instrument or instruments under which such indebtedness is
issued or secured, unless such acceleration is annulled, or such indebtedness is
discharged, or there is deposited in trust a sum of money sufficient to
discharge such indebtedness, within 20 days after written notice has been given
by the Trustee to the Company and Unocal or by holders of at least 25% in
principal amount of the Outstanding Debt Securities of such series to the
Company, Unocal and the Trustee, and (vi) certain events of bankruptcy,
insolvency or reorganization (Section 7.01).
 
    No holder of any Debt Security of a series will have any right to institute
any proceeding with respect to the applicable Indenture or for any remedy
thereunder, unless such holder previously has given to the Trustee written
notice of an Event of Default with respect to such series and unless the holders
of at least 25% in aggregate principal amount of the Debt Securities of that
series at the time outstanding have made written request upon the Trustee, and
have offered reasonable security or indemnity, to institute such proceeding as
trustee under such Indenture, and the Trustee for 60 days shall have failed to
institute such proceeding. However, the right of any holder of any Debt Security
to institute suit for enforcement of any payment of principal of and interest on
such Debt Security on or after the due date expressed in such Debt Security may
not be impaired or affected without such holder's consent (Section 7.04).
 
    The holders of a majority in principal amount of Debt Securities of any
series at the time outstanding may direct the time, method and place of
conducting any proceeding for any remedy available to the Trustee or exercising
any trust or power conferred on the Trustee with respect to Debt Securities of
that series, provided that such holders have offered reasonable security or
indemnity against the costs, expenses and liabilities which might be incurred by
the Trustee in compliance with any such direction and subject to certain other
restrictions (Sections 7.06 and 8.02(d)).
 
    Unocal and the Company will be required to furnish to the Trustee within 120
days after the end of each fiscal year a statement as to their respective
compliance with all conditions and covenants under the Indentures (Sections 4.06
and 5.07).
 
MANDATORY PREPAYMENT
 
    The provisions of each of (i) the $1,400,000,000 Credit and Guarantee
Agreement, dated as of December 12, 1991, as amended as of July 5, 1994, among
the Company and Unocal Canada Limited, as borrowers, Unocal, as guarantor, and a
syndicate of banks, (ii) the $45,000,000 Credit and Guarantee Agreement, dated
as of April 19, 1993, as amended as of September 14, 1994, among Unocal
Netherlands B.V., as borrower, the Company, Unocal, and others, as guarantors,
and The Bank of Nova Scotia, as agent, and (iii) the $250,000,000 Credit and
Guarantee Agreement, dated as of December 15, 1993, among Unocal Thailand,
Ltd.--Thailand Branch, as borrower, the Company and Unocal, as guarantors, and a
syndicate of banks, provide for the termination of the loan commitments
thereunder and require the prepayment of all outstanding loans and all other
amounts owing thereunder in the event (a) any person or group becomes the
beneficial owner of more than 30% of the then outstanding voting stock of
Unocal, otherwise than in a transaction having the approval of the Board of
Directors of Unocal, at least a majority of which are continuing directors, or
(b) continuing directors shall cease to constitute at least a majority of the
Board of Directors of Unocal. The Company or Unocal may include similar or
different mandatory
 
                                       10
<PAGE>
prepayment provisions in other borrowing instruments including, without
limitation, Debt Securities issued in the future. There can be no assurance that
the Company will have the funds available to prepay such amounts if required to
do so under any of these mandatory prepayment provisions.
 
SUBORDINATION
 
    The indebtedness represented by the Subordinated Debt Securities and the
Guarantees of Subordinated Debt Securities will be subordinate and junior in
right of payment to the prior payment in full of all Senior Debt of the Company
or Unocal, as the case may be, whether outstanding on the date of the
Subordinated Indenture or thereafter incurred. "Senior Debt" is defined as (i)
all indebtedness of the Company or Unocal, as the case may be, for borrowed
money, (ii) all indebtedness for borrowed money of others guaranteed by the
Company or Unocal and (iii) any obligation of the Company or Unocal under any
interest rate or currency swap agreement, in each case whether outstanding on
the date of the Indenture or incurred thereafter that is not by its terms
subordinate and junior in right of payment to any other indebtedness of the
Company or Unocal, as the case may be, and, in the case of the Company, includes
all indebtedness at any time evidenced by Senior Debt Securities (Subordinated
Indenture Section 16.09).
 
    In the event (i) of any liquidation, dissolution or other winding up of the
Company or Unocal, or of any receivership, insolvency, bankruptcy, readjustment,
reorganization or other similar proceedings relative to the Company or Unocal or
their respective property, all principal of and any interest due on all Senior
Debt will be paid in full, or provided for, before any principal, sinking fund,
if any, or interest payment is made on the Subordinated Debt Securities, in the
case of the Company, or the Guarantees of Subordinated Debt Securities, in the
case of Unocal, or (ii) that the Subordinated Debt Securities are declared due
and payable because of the occurrence of an Event of Default (under
circumstances such that the preceding clause (i) will not be applicable), the
holders of the Subordinated Debt Securities will be entitled to payment only
after all principal of and any interest due on the Senior Debt has been paid or
has been provided for (Subordinated Indenture Section 16.01).
 
    By reason of such subordination, creditors of the Company who are holders of
Senior Debt Securities may recover more, ratably, than holders of Subordinated
Debt Securities.
 
CONVERSION AND EXCHANGE
 
    The terms, if any, on which Offered Debt Securities are convertible into or
exchangeable for Unocal Preferred Stock or Unocal Common Stock will be set forth
in the Prospectus Supplement relating thereto. Such terms may include provisions
for conversion or exchange, either mandatory, at the option of the holder or at
the option of the Company.
 
FORM, EXCHANGE, REGISTRATION AND TRANSFER
 
    The Debt Securities may be issued in fully registered form without coupons,
in a form registered as to principal only with or without bearer coupons
("Registered Securities") or in bearer form with or without coupons ("Bearer
Securities") or any combination thereof. Debt Securities may also be issued in
whole or in part, in the form of one or more temporary or permanent global
securities (each a "Global Security"). Unless otherwise specified in the
applicable Prospectus Supplement relating to the Offered Debt Securities, the
Debt Securities will be only Registered Securities. The Debt Securities
denominated in United States Dollars will be issued, unless otherwise set forth
in the applicable Prospectus Supplement relating to the Offered Debt Securities,
in denominations of $1,000 for Registered Securities and in denominations of
$5,000 for Bearer Securities, and in any integral multiple of such denominations
(Section 2.02). See, however, "Limitations on the Issuance of Bearer Securities"
below. One or more Global Securities will be issued in a denomination or
aggregate denominations equal to the aggregate principal amount of Outstanding
Debt Securities of the series to be represented by such Global Security or
Securities. The Prospectus Supplement relating to a series of Debt Securities
denominated in a foreign or composite currency will specify the denomination
thereof.
 
                                       11
<PAGE>
    Registered Securities of any series (other than a Global Security, except as
set forth below) will be exchangeable for other Registered Securities of the
same series and of a like aggregate principal amount and tenor of different
authorized denominations. In addition, if Debt Securities of any series are
issuable as both Registered Securities and Bearer Securities, at the written
request of the holder, and subject to the terms of the applicable Indenture,
Bearer Securities (with all unmatured coupons, except as provided below, and all
mature coupons in default) of such series will be exchangeable into Registered
Securities of the same series of any authorized denominations and of a like
aggregate principal amount and tenor. No Bearer Securities will be delivered in
the United States. Bearer Securities with coupons appertaining thereto
surrendered in exchange for Registered Securities between a Regular Record Date,
or, in certain circumstances a Special Record Date, and the relevant date for
payment of interest must be surrendered without the coupon relating to such date
for payment of interest and such interest will not be payable in respect of the
Registered Security issued in exchange for such Bearer Security, but will be
payable only to the holder of such coupon when due in accordance with the terms
of the applicable Indenture. Unless otherwise stated in a Prospectus Supplement,
Registered Securities will not be exchangeable into Bearer Securities. If a
holder elects to receive a definitive Bearer Security, rather than hold an
interest in a permanent global Bearer Security, then, at the option of the
Company, such holder must pay to the Company a service charge and a
proportionate share of the cost of printing such definitive Bearer Security
(Section 2.05).
 
    Debt Securities may be presented for exchange as provided above, and
Registered Securities (other than a Global Security) may be presented for
registration of transfer (with the form of transfer endorsed thereon duly
executed), at the office of the Security Registrar or at the office of any
transfer agent designated by the Company for such purpose with respect to any
series of Debt Securities and specified in the applicable Prospectus Supplement,
upon payment of any required service charges and taxes and other governmental
charges. The holders of the Debt Securities will be required to pay all service
charges for the exchange or transfer of any Debt Security, except the Company
shall pay for such service charges (i) for the transfer from a temporary global
Debt Security to any other form of Debt Security, (ii) if the Debt Securities
are listed on a stock exchange that requires the issuer to pay such charges as a
condition to listing or (iii) if the applicable Prospectus Supplement otherwise
specifies. Such transfer or exchange will be effected once the Security
Registrar or such transfer agent, as the case may be, is satisfied with the
document of title and identity of the person making the request. Bearer
Securities will be transferable by delivery.
 
    The Company has appointed the Senior Trustee under the Senior Indenture, and
will appoint the Subordinated Trustee under the Subordinated Indenture, as
Security Registrar (Section 2.05). At the date of this Prospectus, the Corporate
Trust Office of the Senior Trustee is located at 300 South Grand Avenue, 4th
Floor, Los Angeles, California 90071. If the identity or address of the Senior
Trustee changes, the corrected information will appear in the applicable
Prospectus Supplement, as appropriate. The identity and address of the
Subordinated Trustee will appear in the applicable Prospectus Supplement. If the
applicable Prospectus Supplement specifies any transfer agents in addition to
the Security Registrar with respect to any series of Debt Securities, the
Company may at any time rescind the designation of any such transfer agent or
approve a change in the location through which any such transfer agent acts,
except that, if Debt Securities of a series are issuable only as Registered
Securities, the Company will be required to maintain a transfer agent in each
Place of Payment for such series and, if Debt Securities of a series are
issuable as Bearer Securities, the Company will be required to maintain (in
addition to the Security Registrar) a transfer agent in a Place of Payment for
such series located outside the United States. The Company may at any time
designate additional transfer agents with respect to any series of Debt
Securities (Section 4.02).
 
    In the event of any redemption in part, the Company shall not be required
to: (i) issue, register the transfer or exchange of Debt Securities of any
series during a period beginning at the opening of 15 Business Days before any
selection of Debt Securities of that series to be redeemed and ending at the
close
 
                                       12
<PAGE>
of business on (a) the day of mailing of the relevant notice of redemption, if
Debt Securities of the series are issuable only as Registered Securities, (b)
the day of the first publication of the relevant notice of redemption, if Debt
Securities of the series are issuable only as Bearer Securities, or (c) the day
of mailing of the relevant notice of redemption, if Debt Securities of the
series are issuable as Registered Securities and Bearer Securities and there is
no publication; (ii) register the transfer or exchange of any Registered
Security, or portion thereof, called for redemption, except the unredeemed
portion of any Registered Security being redeemed in part; or (iii) exchange any
Bearer Security called for redemption, except to exchange such Bearer Security
for a Registered Security of that series and like tenor which is simultaneously
surrendered for redemption (Section 2.05).
 
PAYMENT AND PAYING AGENTS
 
    Payment of principal of, and any interest on, Registered Securities, unless
otherwise specified in the applicable Prospectus Supplement, will be made at the
office of the Paying Agent or Paying Agents as the Company may designate from
time to time, except that at the option of the Company payment of any interest
may be made by check mailed to the address of the person entitled thereto as
such address shall appear in the Security Register (Section 2.11). Payment of
any installment of interest on Registered Securities will be made to the person
in whose name such Registered Security is registered at the close of business on
the Regular Record Date for such interest (Section 2.09), except as otherwise
specified in the applicable Prospectus Supplement.
 
    Payment of principal of, and any interest on, Bearer Securities will be
payable in United States dollars, unless a different currency is designated in
the applicable Prospectus Supplement, subject to any applicable laws and
regulations, at the offices of such Paying Agents outside the United States as
the Company may designate from time to time. Payment of interest on Bearer
Securities with coupons appertaining thereto on any Interest Payment Date will
be made only against surrender of the coupon relating to such Interest Payment
Date, unless otherwise indicated in the applicable Prospectus Supplement
(Sections 2.11 and 4.02). No payment with respect to any Bearer Security will be
made at the Corporate Trust Office of the Trustee or any office or agency of the
Company in the United States or by check mailed to any address in the United
States or by transfer to an account maintained in the United States.
Notwithstanding the foregoing, payments of principal of, and any interest on,
Bearer Securities denominated and payable in United States Dollars will be made
at the office of the Company's Paying Agent in New York City, if (but only if)
payment of the full amount thereof in United States Dollars at all offices or
agencies outside the United States is illegal or effectively precluded by
exchange controls or other similar restrictions (Section 4.02).
 
    The Company has designated the New York City Corporate Trust Office of the
Senior Trustee, and will designate the New York City Corporate Trust Office of
the Subordinated Trustee, as the sole Paying Agent for payments with respect to
Offered Debt Securities that are issuable as Registered Securities, and as the
Paying Agent in New York City for payments with respect to Offered Debt
Securities (subject to the limitations described above in the case of Bearer
Securities) that are issuable solely as Bearer Securities or as both Registered
Securities and Bearer Securities. Any Paying Agents outside the United States
and any other Paying Agents in the United States initially designated by the
Company for the Offered Debt Securities will be named in the applicable
Prospectus Supplement. The Company may at any time designate additional Paying
Agents or rescind the designation of any Paying Agent or approve a change in the
office through which any Paying Agent acts. However, the Company will be
required to maintain a Paying Agent in each Place of Payment for Debt Securities
of each series that is issuable solely as Registered Securities, and the Company
will be required to maintain for each series of Bearer Securities a Paying Agent
(i) in New York City for payments with respect to any Registered Securities of
the series (and for payments with respect to Bearer Securities of the series in
the circumstances described above, but not otherwise), (ii) in a place of
payment located outside the United States where Debt Securities of such series
and any coupons appertaining thereto may be presented and surrendered for
payment; and (iii) each
 
                                       13
<PAGE>
place outside the United States required by any stock exchange on which Debt
Securities of such series are listed (Section 4.02).
 
    All monies paid by the Company to a Paying Agent for the payment of
principal of, and any interest on, any Debt Securities that remain unclaimed at
the end of two years after such principal or interest has become due and payable
will be repaid to the Company and the holder of such Debt Security or any coupon
appertaining thereto will thereafter look only to the Company or Unocal for
payment thereof (Section 13.05).
 
GLOBAL SECURITIES
 
    The Offered Debt Securities may be issued in whole or in part in the form of
one or more Global Securities that will be deposited with, or on behalf of, a
depositary (the "Depositary") identified in the applicable Prospectus
Supplement. Global Securities may be issued in either registered or bearer form
and in either temporary or definitive form. Unless and until it is exchanged in
whole or in part for Debt Securities in definitive form, a Global Security may
not be transferred except as a whole by the Depositary for such Global Security
to a nominee of such Depositary or by a nominee of such Depositary to such
Depositary or another nominee of such Depositary or by such Depositary or any
such nominee to a successor of such Depositary or a nominee of such successor
(Sections 2.03 and 2.05).
 
    The specific terms of the depositary arrangement with respect to any Offered
Debt Securities will be described in the applicable Prospectus Supplement. The
Company anticipates that the following provisions will apply to all depositary
arrangements.
 
    Upon the issuance of a Global Security, the Depositary for such Global
Security will credit, on its book-entry registration and transfer system, the
respective principal amounts of the Debt Securities represented by such Global
Security to the accounts of institutions that have accounts with such Depositary
("Participants"). The accounts to be credited shall be designated by the
underwriters of such Debt Securities, by certain agents of the Company or by the
Company, if such Debt Securities are offered and sold directly by the Company.
Ownership of beneficial interests in a Global Security will be limited to
Participants or persons that may hold interests through Participants. Ownership
of beneficial interests in such Global Security will be shown on, and the
transfer of that ownership will be effected only through, records maintained by
the Depositary for such Global Security or by Participants or by persons that
hold through Participants. The laws of some jurisdictions require that certain
purchasers of securities take physical delivery of such securities in definitive
form. Such ownership limits and such laws may impair the ability to transfer
beneficial interests in a Global Security.
 
    So long as the Depositary for a Global Security, or its nominee, is the
owner of such Global Security, such Depositary or such nominee, as the case may
be, will be considered the sole owner or holder of the Debt Securities
represented by such Global Security for all purposes under the Indenture
governing such Debt Securities. Except as set forth below, owners of beneficial
interests in a Global Security will not be entitled to have Debt Securities of
the series represented by such Global Security registered in their names, will
not receive or be entitled to receive physical delivery of Debt Securities of
such series in definitive form and will not be considered the owners or holders
thereof under the Indenture governing such Debt Securities.
 
    Subject to the restrictions discussed under "Limitations on the Issuance of
Bearer Securities" below, principal and interest payments on Debt Securities
registered in the name of or held by a Depositary or its nominee will be made to
the Depositary or its nominee, as the case may be, as the registered owner or
the holder of the Global Security representing such Debt Securities. None of the
Company, Unocal, the Trustee for such Debt Securities, any paying agent or the
Security Registrar for such Debt Securities will have any responsibility or
liability for any aspect of the records relating to or payments made on account
of beneficial ownership interests in a Global Security for such Debt Securities
or for maintaining, supervising or reviewing any records relating to such
beneficial ownership interests.
 
                                       14
<PAGE>
    The Company expects that the Depositary for Debt Securities of a series,
upon receipt of any payment of principal or interest in respect of a definitive
Global Security, will immediately credit Participants' accounts with payments in
amounts proportionate to their respective beneficial interests in the principal
amount of such Global Security as shown on the records of such Depositary. The
Company also expects that payments by Participants to owners of beneficial
interests in such Global Security held through such Participants will be
governed by standing instructions and customary practices, as is now the case
with securities held for the accounts of customers in bearer form or registered
in "street name," and will be the responsibility of such participants.
 
    If a Depositary for Debt Securities of a series is at any time unwilling or
unable to continue as Depositary and a successor Depositary is not appointed by
the Company within 90 days, the Company and Unocal will issue Debt Securities of
such series in definitive form in exchange for the Global Security or Securities
representing the Debt Securities of such series. In addition, the Company may at
any time and in its sole discretion determine not to have any Debt Securities of
a series represented by one or more Global Securities and, in such event, will
issue Debt Securities of such series in definitive form in exchange for the
Global Security or Securities representing such Debt Securities. Further, an
owner of a beneficial interest in a Global Security representing Debt Securities
of such series may, under certain circumstances and on terms acceptable to the
Company and the Depositary for such Global Security, receive Debt Securities of
such series in definitive form. In any such instance, an owner of a beneficial
interest in a Global Security will be entitled to physical delivery in
definitive form of Debt Securities of the series represented by such Global
Security equal in principal amount to such beneficial interest and to have such
Debt Securities registered in its name (if the Debt Securities of such series
are issuable as Registered Securities). Unless otherwise specified by the
Company, Debt Securities of such series so issued in definitive form will be
issued (a) as Registered Securities in denominations of $1,000 and integral
multiples thereof, if the Debt Securities of such series are issuable as
Registered Securities; (b) as Bearer Securities in the denominations of $5,000,
if the Debt Securities of such series are issuable as Bearer Securities or (c)
as either Registered or Bearer Securities in such denominations, if the Debt
Securities of such series are issuable in either form (Section 2.05). See,
however, "Limitations on the Issuance of Bearer Securities" below for a
description of certain restrictions on the issuance of a Bearer Security in
definitive form in exchange for an interest in a Global Security.
 
MEETINGS, MODIFICATION AND WAIVER
 
    MODIFICATION OF INDENTURES.  The Senior Indenture provides, and the
Subordinated Indenture will provide, that the Company, Unocal and the Trustee
thereunder may, without the consent of any holders of Debt Securities, enter
into supplemental indentures for the purposes, among other things, of adding to
the Company's or Unocal's covenants, adding additional Events of Default,
establishing the form or terms of Debt Securities or curing ambiguities or
inconsistencies in such Indenture or making other provisions; provided such
action shall not adversely affect the interests of the holders of any series of
Debt Securities in any material respect (Section 11.01). In addition,
modifications and amendments of each Indenture may be made by the Company and
Unocal and the Trustee with the consent of the holders of not less than a
majority in aggregate principal amount of the Debt Securities then outstanding
of each series affected by such modification or amendment; provided, however,
that no such modification or amendment may, without the consent of the holder of
each Debt Security then outstanding that is affected thereby, (a) change the
Stated Maturity of the principal of, or any installment of principal of or
interest on any Debt Security, (b) reduce the principal amount of or interest on
any Debt Security, (c) change any obligation to pay additional amounts, (d)
reduce the amount of principal of an Original Issue Discount Security payable
upon acceleration of the Maturity thereof, (e) change the Place of Payment or
the currency or currency unit in which any Debt Security or interest thereon is
payable, (f) impair the right to institute suit for the enforcement of any
payment on or with respect to any Debt Security, (g) reduce the percentage in
principal amount of Debt Securities then outstanding of any series, the consent
of whose holders is required for modification or amendment of the applicable
Indenture or for any waiver of compliance with
 
                                       15
<PAGE>
certain provisions of the Indenture or for waiver of certain defaults, (h)
change any obligation of the Company to maintain an office or agency in the
places and for the purposes required by an Indenture, (i) if the Debt Securities
are convertible into any other security of the Company or Unocal, make any
change that would materially adversely affect the right to convert such Debt
Securities, or (j) modify any of the above provisions. If the Debt Securities of
any series are issuable upon the exercise of Debt Warrants, then each holder of
a Debt Warrant with respect to such series shall be treated as a holder of such
Debt Securities in the amount issuable upon exercise of such Debt Warrant for
purposes of voting under Section 11.02 of the Indenture (Sections 9.04 and
11.02).
 
    WAIVER OF DEFAULT.  The holders of a majority in aggregate principal amount
of the Debt Securities then outstanding of each series may, on behalf of the
holders of all the Debt Securities of that series, waive, insofar as that series
is concerned, compliance by Unocal with certain restrictive provisions of the
applicable Indenture (Section 5.11). The holders of a majority in aggregate
principal amount of the Debt Securities then outstanding of each series may, on
behalf of all holders of Debt Securities of that series and any coupons
appertaining thereto, waive any past default under the Indenture with respect to
Debt Securities of that series, except a default (a) in the payment of principal
of or any interest on any Debt Security of such series and (b) in respect of a
covenant or provision of the Indenture which cannot be modified or amended
without the consent of the holder of each Debt Security then outstanding of such
series affected (Section 7.06).
 
    CALCULATING OUTSTANDING PRINCIPAL.  The Senior Indenture provides, and the
Subordinated Indenture will provide, that in determining whether the holders of
the requisite principal amount of the Debt Securities that are outstanding have
given any request, demand, authorization, direction, notice, consent or waiver
thereunder or are present at a meeting of holders of Debt Securities for quorum
purposes, (i) the principal amount of an Original Issue Discount Security that
will be deemed to be outstanding will be the amount of the principal thereof
that would be due and payable as of the date of such determination upon
acceleration of the Maturity thereof, and (ii) the principal amount of a Debt
Security denominated in a foreign currency or currency unit will be deemed to be
that amount of United States dollars that could be obtained for such principal
amount on the basis of the spot rate of exchange for such foreign currency or
currency unit as determined by the Company or an Exchange Rate Agent up to ten
days before the date of the action by the holders (Section 9.04).
 
    MEETINGS AND VOTING.  The Senior Indenture contains, and the Subordinated
Indenture will contain, a provision for convening meetings of the holders of
Debt Securities of a series, including Debt Securities issuable as Bearer
Securities (Section 10.01). A meeting may be called at any time by the Trustee,
and upon request, by the Company, Unocal or the holders of at least 25% in
principal amount of the Debt Securities then outstanding of such series, in any
such case upon notice given in accordance with "Notices" below (Sections 10.02
and 10.03). Except as described above under "Modifications of Indentures" and
"Waiver of Default," a resolution presented at a meeting or reconvened meeting
at which a quorum of the holders of Debt Securities then outstanding of the
applicable series is present may be adopted by the affirmative vote of the
lesser of (i) the holders of a majority in principal amount of the Debt
Securities then outstanding of such series, or (ii) the holders of 66 2/3% in
principal amount of the Debt Securities then outstanding of such series
represented and voting at the meeting; provided, however, that if any consent,
waiver, or other action which the applicable Indenture expressly provides may be
made, given or taken by the holders of a specified percentage, which is less
than a majority of the principal amount of the Debt Securities then outstanding
of a series, such action may be adopted at a meeting or reconvened meeting at
which a quorum is present by the affirmative vote of the lesser of (a) the
holders of such specified percentage in principal amount of the Debt Securities
then outstanding of that series or (b) a majority in principal amount of Debt
Securities then outstanding of such series represented and voting at the
meeting. Any resolution passed or decision taken at any meeting of holders of
Debt Securities of any series duly held in accordance with the Indenture will be
binding on all holders of Debt Securities of that series and the related coupons
whether or not present or represented at the meeting.
 
                                       16
<PAGE>
    The quorum at a meeting of the holders of a series of Debt Securities will
be persons holding or representing a majority in principal amount of the Debt
Securities then outstanding of a series, unless otherwise specified in a
Prospectus Supplement (Section 10.08).
 
    The record date for purposes of determining the identity of holders entitled
to vote regarding, or consent to, actions by the Trustee and certain waivers
will be the later of (i) thirty (30) days prior to the first solicitation of
such consent or (ii) the date of the most recent list of holders of securities
furnished to the Trustee prior to such solicitation.
 
NOTICES
 
    Except as otherwise provided in the applicable Indenture, notices to holders
of Bearer Securities will be given by publication at least once in a newspaper
published on a Business Day in New York City and London and in such other city
or cities as may be required with respect to such Bearer Securities and will be
mailed to such persons whose names and addresses were previously filed with the
Trustee under the applicable Indenture, within the time prescribed for the
giving of such notice. Notices to holders of Registered Securities will be given
by mail to the address of such holders as they appear in the Security Register
(Section 1.04).
 
TITLE
 
    Title to any Bearer Securities (including Bearer Securities in permanent
global bearer form) and any coupons appertaining thereto will pass by delivery.
The Company, Unocal, the appropriate Trustee and any agent of the Company or
such Trustee may treat the bearer of any Bearer Securities, the bearer of any
coupon and the registered owner of any Registered Security as the absolute owner
thereof (whether or not such Debt Security or coupon is overdue and
notwithstanding any notice to the contrary) for the purpose of making payment
and for all the other purposes (Section 2.07).
 
DEFEASANCE
 
    Unless otherwise indicated in the applicable Prospectus Supplement, the
obligations of the Company and Unocal with respect to the payment of the
principal of and interest on the Offered Debt Securities and their respective
obligations under Sections 5.01, 5.02, 5.03, 5.04, 5.05, 5.08, 5.09, 5.11, 12.01
and 12.02 of the Indenture will be terminated if: (i) the Company irrevocably
deposits or causes to be deposited with the appropriate Trustee, under the terms
of an escrow trust agreement in form and substance satisfactory to the
appropriate Trustee, as trust funds pledged as security for, and dedicated
solely to, the benefit of the holders of the Offered Debt Securities, (a) money
or (b) in the case of Offered Debt Securities and coupons denominated in United
States Dollars, U.S. Government Obligations (as defined in Section 13.04), and
in the case of Debt Securities and coupons denominated in a foreign currency,
Foreign Government Securities (as defined in Section 13.04), which through the
payment of interest thereon and principal thereof in accordance with their terms
will provide money or (c) a combination of (a) and (b), in each case in an
amount sufficient to pay in the currency or currency unit in which the Offered
Debt Securities are payable all the principal of and interest on the Offered
Debt Securities on the dates such payments are due in accordance with the terms
of the Offered Debt Securities; and (ii) the Company furnishes to the
appropriate Trustee a ruling by the Internal Revenue Service, in form and
substance satisfactory to such Trustee, or an Opinion of Counsel, in form and
substance satisfactory to the appropriate Trustee, to the effect, in either
case, that the holders of such Offered Debt Securities (a) will not recognize
income, gain or loss for Federal income tax purposes as a result of the
Company's exercise of the defeasance provisions of the Indenture and (b) will be
subject to Federal income tax in the same amount, in the same manner and at the
same time as would have been the case if the Company had not exercised its
defeasance rights under the Indenture (Section 13.03).
 
                                       17
<PAGE>
THE TRUSTEES
 
    A Trustee may resign or be removed with respect to one or more series of
Debt Securities and a successor Trustee may be appointed by the Company to act
with respect to such series (Section 8.10). In the event that two or more
Persons are acting as Trustee with respect to different series of Debt
Securities under one of the Indentures, each such Trustee will be deemed to be a
Trustee of a trust under the applicable Indenture, separate and apart from the
trust administered by any other such Trustee, and any action described herein to
be taken by the "Trustee" may then be taken by each such Trustee with respect
to, and only with respect to, the one or more series of Debt Securities for
which it is Trustee (Section 8.11).
 
    The initial Senior Trustee is Chemical Trust Company of California, Los
Angeles, California. The identity of the initial Subordinated Trustee has yet to
be determined. Chemical Bank, an affiliate of the Senior Trustee, is a lending
bank and one of the agents under the $1,400,000,000 Credit and Guarantee
Agreement, as amended, referred to above under "Mandatory Prepayment" and the
Company maintains various accounts and conducts other normal banking
transactions with Chemical Bank. The Senior Trustee is the transfer agent and
registrar for the Unocal Common Stock and $3.50 Convertible Preferred Stock and
is the Rights Agent under the Rights Agreement, dated as of January 29, 1990,
referred to below under "Description of the Common Stock--Rights to Purchase
Series A Preferred Stock." Chemical Bank is the administrator of the Unocal
Dividend Reinvestment and Common Stock Purchase Plan. Unocal and the Company may
in the future maintain other banking relationships with the Senior Trustee and
Chemical Bank in the ordinary course of business and may do the same with the
Subordinated Trustee.
 
GOVERNING LAW
 
    The Indentures, the Debt Securities, the Guarantees, and the coupons will be
governed by, and construed in accordance with, the laws of the State of New York
(Section 15.05).
 
                       DESCRIPTION OF THE PREFERRED STOCK
 
    The following description of Unocal Preferred Stock sets forth certain
general terms and provisions of the series of Unocal Preferred Stock to which
any Prospectus Supplement may relate. The specific terms of an offered series of
Unocal Preferred Stock will be described in the Prospectus Supplement relating
to such series. If so indicated in the Prospectus Supplement relating thereto,
the terms of any such series of Unocal Preferred Stock may differ from the terms
set forth below. The description of Unocal Preferred Stock set forth below and
the description of the terms of an offered series of Unocal Preferred Stock set
forth in the Prospectus Supplement relating thereto do not purport to be
complete and are qualified in their entirety by reference to Unocal's
Certificate of Incorporation, as amended (the "Certificate of Incorporation"),
and the Certificate of Designations relating to such offered series of Unocal
Preferred Stock, which will be filed with the Commission and incorporated by
reference as an exhibit to the Registration Statement of which this Prospectus
is a part at or prior to the time of the sale of such offered series.
 
    Under Unocal's Certificate of Incorporation, Unocal's Board of Directors is
authorized, without further stockholder action, to provide for the issuance of
up to 100,000,000 shares of preferred stock, $0.10 par value per share, in one
or more series, with or without voting powers, and with such designations,
preferences and relative, participating, optional or other special rights, and
qualifications, limitations or restrictions, as the Board of Directors shall
determine. As of the date of this Prospectus, Unocal has outstanding 10,250,000
shares of $3.50 Convertible Preferred Stock ("$3.50 Preferred Stock"). So long
as any shares of $3.50 Preferred Stock are outstanding, the preferences,
privileges and voting powers, if any, of the shares of Unocal Preferred Stock of
any series, and the restrictions or qualifications thereof, shall be subject to
the preferences, privileges and voting powers, if any, of the shares of $3.50
Preferred Stock. See "Description of Outstanding Preferred Stock." In addition,
Unocal has reserved for issuance and designated 3,000,000 shares of preferred
stock as Series A Junior Participating Cumulative Preferred Stock in
 
                                       18
<PAGE>
connection with the Rights Plan described below under "Description of the Common
Stock--Rights to Purchase Series A Preferred Stock."
 
GENERAL
 
    The applicable Prospectus Supplement will set forth the following specific
terms regarding the series of Unocal Preferred Stock offered thereby: (i) the
designation, number of shares and liquidation preference per share; (ii) the
initial public offering price; (iii) the dividend rate or rates, if any, or the
method of determining the dividend rate or rates; (iv) the index, if any, upon
which the amount of dividends is to be determined; (v) the dates on which
dividends will accrue and be payable and the designated record dates for
determining the holders entitled to such dividends; (vi) any redemption or
sinking fund provisions; (vii) any conversion or exchange provisions; (viii) any
provisions for the issuance of global securities; (ix) the currency (which may
be composite currency) in which liquidation preferences, redemption prices and
dividends shall be payable, if other than United States dollars; (x) voting
rights, if different from those described under "--Voting Rights"; and (xi) any
additional terms, preferences or rights.
 
    The shares of Unocal Preferred Stock will, when issued, be fully paid and
nonassessable and will have no preemptive rights.
 
    The transfer agent, registrar, dividend disbursing agent, redemption agent
and, if applicable, conversion agent for the offered series of Unocal Preferred
Stock will be specified in the applicable Prospectus Supplement relating
thereto.
 
DIVIDENDS
 
    The holders of the Unocal Preferred Stock of each series will be entitled to
receive, when, as and if declared by the Board of Directors of Unocal, out of
funds legally available therefor, cumulative or non-cumulative cash or other
dividends at such rate or rates and on such dates as will be set forth in the
applicable Prospectus Supplement. Such rates may be fixed or variable or both.
If variable, the formula used for determining the dividend rate for each
dividend period will be set forth in the Prospectus Supplement. Dividends will
be payable to the holders of record as they appear on the stock register of
Unocal on such record dates, not more than sixty (60) days nor less than ten
(10) days preceding the payment dates thereof, as will be fixed by the Board of
Directors of Unocal. If the Board of Directors of Unocal fails to declare a
dividend payable on a dividend payment date on any series of Unocal Preferred
Stock for which dividends are noncumulative ("Noncumulative Preferred Stock"),
then the holders of such series of Noncumulative Preferred Stock will have no
right to receive a dividend in respect of the dividend period ending on such
dividend payment date, and Unocal will have no obligation to pay a dividend for
such period, whether or not dividends on such series are declared payable on any
future dividend payment dates. Dividends payable on any series of Unocal
Preferred Stock for any period less than a full dividend period will be computed
on the basis of a 360-day year consisting of twelve 30-day months.
 
    If the offered series of Unocal Preferred Stock ranks junior to or on a
parity with the $3.50 Preferred Stock as to dividends, no full dividends may be
declared or paid or set apart for payment on such offered series of Unocal
Preferred Stock for any period unless full cumulative dividends have been or
contemporaneously are declared and paid, or declared and a sum sufficient for
the payment thereof set apart for such payment, on the $3.50 Preferred Stock for
all dividend payment periods terminating on or prior to the date of the payment
of such full cumulative dividends. If the offered series of Unocal Preferred
Stock ranks on a parity with the $3.50 Preferred Stock and dividends are not
paid in full on the $3.50 Preferred Stock, then all dividends declared upon all
outstanding shares of $3.50 Preferred Stock and shares of such offered series of
Unocal Preferred Stock will be declared pro rata so that the amounts of
dividends declared per share on the $3.50 Preferred Stock and such offered
series of Unocal Preferred Stock will in all cases bear to each other the same
ratio that accrued and unpaid dividends per share on the shares of $3.50
Preferred Stock and such offered Unocal Preferred Stock bear to each other.
 
                                       19
<PAGE>
    If the offered series of Unocal Preferred Stock ranks junior to or on a
parity with the $3.50 Preferred Stock, then unless full cumulative dividends on
all outstanding shares of $3.50 Preferred Stock have been paid or declared and
set aside for payment for all past dividend payment periods, no dividend or
distribution (other than a dividend or distribution in Unocal Common Stock or in
any other capital stock of Unocal ranking junior to the $3.50 Preferred Stock as
to dividends and upon liquidation and other than as provided in the preceding
paragraph) may be declared or paid or set apart for payment on the offered
series of Unocal Preferred Stock nor may the offered series of Unocal Preferred
Stock be redeemed, purchased or otherwise acquired for any consideration (or any
moneys be paid to or made available for a sinking fund for the redemption of any
shares of any such stock) by Unocal (except by conversion into or exchange for
capital stock of Unocal ranking junior to the $3.50 Preferred Stock as to
dividends and upon liquidation). These restrictions will not prevent Unocal from
making contributions to, or purchasing capital stock in connection with, its
employee benefit plans and dividend reinvestment plan or from redeeming rights
pursuant to its Rights Plan, described below under "Description of the Common
Stock--Rights to Purchase Series A Preferred Stock."
 
REDEMPTION
 
    The offered series of Unocal Preferred Stock may be redeemable at the option
of Unocal and may be subject to mandatory redemption pursuant to a sinking fund
or otherwise, in each case upon the terms, on the date or dates and at the
redemption price or prices set forth in the applicable Prospectus Supplement. If
fewer than all shares of the offered series of Unocal Preferred Stock are to be
redeemed, the shares to be redeemed will be selected by Unocal pro rata or by
lot, by any other method determined by the Board of Directors to be equitable,
or by any method set forth in the applicable Prospectus Supplement.
 
    If any dividends on shares of the offered series of Unocal Preferred Stock
are in arrears, no shares of Unocal Common Stock or shares of capital stock
ranking junior to or on parity with the offered series of Unocal Preferred Stock
may be redeemed and no shares of such offered series of Unocal Preferred Stock
may be redeemed unless all outstanding shares of such series are simultaneously
redeemed, and Unocal may not purchase or otherwise acquire any shares of such
series; provided, however, that the foregoing shall not prevent the purchase or
acquisition of shares of such series pursuant to a purchase or exchange offer
made on the same terms to holders of all outstanding shares of such series.
 
    Notice of redemption will be given by mailing the same to each record holder
of the shares to be redeemed to the respective addresses of such holders as the
same shall appear on Unocal's stock register. Each such notice will state: (i)
the redemption date; (ii) the number of shares and series of Unocal Preferred
Stock to be redeemed; (iii) the redemption price and the manner in which such
redemption price is to be paid and delivered; (iv) the place or places where
certificates for such shares of Unocal Preferred Stock are to be surrendered for
payment of the redemption price; and (v) that dividends on the shares to be
redeemed will cease to accrue on such redemption date. If fewer than all shares
of any series of Unocal Preferred Stock held by any holder are to be redeemed,
the notice mailed to such holder will also specify the number of shares to be
redeemed from such holder.
 
    If notice of redemption has been given, from and after the redemption date
for the shares of the series of Unocal Preferred Stock called for redemption
(unless default shall be made by Unocal in providing money for the payment of
the redemption price of the shares so called for redemption), dividends on the
shares of Unocal Preferred Stock so called for redemption will cease to accrue,
any right to convert the shares of Unocal Preferred Stock will terminate, such
shares will no longer be deemed to be outstanding, and all rights of the holders
thereof as stockholders of Unocal (except the right to receive the redemption
price, without interest) will cease. Upon surrender in accordance with such
notice of the certificates representing any shares so redeemed (properly
endorsed or assigned for transfer, if the notice shall so state), the redemption
price set forth above will be paid out of funds provided by Unocal. If fewer
than all of the shares represented by any such certificate are redeemed, a new
certificate will be issued representing the unredeemed shares without cost to
the holder thereof.
 
                                       20
<PAGE>
LIQUIDATION PREFERENCE
 
    The applicable Prospectus Supplement will set forth the specific liquidation
preference of the offered series of Unocal Preferred Stock.
 
    If the offered series of Unocal Preferred Stock ranks on a parity with the
$3.50 Preferred Stock, then upon any voluntary or involuntary liquidation,
dissolution or winding up of Unocal, the holders of shares of such offered
series of Unocal Preferred Stock and of $3.50 Preferred Stock will be entitled
to receive out of the assets of Unocal available for distribution to
stockholders, before any distribution of assets is made to or set apart for the
holders of Unocal Common Stock or of any other shares of capital stock of Unocal
ranking as to such a distribution junior to the shares of such series, with
respect to the offered series of Unocal Preferred Stock, an amount described in
the Prospectus Supplement relating to such offered series of Unocal Preferred
Stock, and with respect to $3.50 Preferred Stock, an amount equal to the
liquidation value of such shares. See "--Description of Outstanding Preferred
Stock." If the offered series of Unocal Preferred Stock ranks on a parity with
the $3.50 Preferred Stock, and upon any voluntary or involuntary dissolution,
liquidation or winding up of Unocal, the amounts payable with respect to the
liquidation preference of the $3.50 Preferred Stock and the offered series of
Unocal Preferred Stock are not paid in full, then the holders of $3.50 Preferred
Stock and the offered series of Unocal Preferred Stock will share ratably in any
such distribution of assets of Unocal in proportion to the full distributable
amounts to which they are entitled. After payment of the full amount of the
liquidating distributions to which they are entitled, the holders of $3.50
Preferred Stock and the offered series of Unocal Preferred Stock will have no
right or claim to any of the remaining assets of Unocal. Neither the sale of all
or substantially all of the property or business of Unocal (other than in
connection with the winding up of its business), nor the merger or consolidation
of Unocal into or with any other corporation will be deemed to be a dissolution,
liquidation or winding up, voluntary or involuntary, of Unocal.
 
    Unocal conducts substantially all of its operations through the Company. The
right of Unocal, and hence the right of creditors and stockholders of Unocal, to
participate in any distribution of assets of any subsidiary (including the
Company) upon its liquidation or reorganization or otherwise is necessarily
subject to the prior claims of creditors of the subsidiary, except to the extent
that claims of Unocal itself as a creditor of the subsidiary may be recognized.
 
CONVERSION AND EXCHANGE
 
    The terms, if any, on which shares of any offered series of Unocal Preferred
Stock are convertible into or exchangeable for Unocal Common Stock will be set
forth in the Prospectus Supplement relating thereto. Such terms may include
provisions for conversion or exchange, either mandatory, at the option of the
holder, or at the option of Unocal.
 
VOTING RIGHTS
 
    Except as indicated below or in the Prospectus Supplement relating to a
particular offered series of Unocal Preferred Stock, or except as expressly
required by applicable law, the holders of Unocal Preferred Stock will not be
entitled to vote.
 
    On matters on which holders of such offered series and holders of any other
series of Unocal Preferred Stock are entitled to vote as a single class, each
full share of any series of Unocal Preferred Stock shall be entitled to one
vote. Therefore, the voting power of such series will depend on the number of
shares in such series, not the liquidation preference or initial offering price
of the shares of such series of the Unocal Preferred Stock.
 
    If the equivalent of six quarterly dividends (whether or not consecutive)
payable on any offered series of Unocal Preferred Stock, $3.50 Preferred Stock
or any other series of Unocal Preferred Stock are in default, the number of
directors of Unocal will be increased by two and the holders of all outstanding
 
                                       21
<PAGE>
shares of Unocal Preferred Stock, $3.50 Preferred Stock, and all other
outstanding shares of preferred stock having similar voting rights, voting as a
single class without regard to series and with no cumulative voting, to the
exclusion of the holders of Unocal Common Stock, will be entitled to elect those
two additional directors, who shall serve until all dividends in default have
been paid or declared and set apart for payment.
 
    So long as any shares of Unocal Preferred Stock and $3.50 Preferred Stock
remain outstanding, Unocal shall not, without the consent of the holders of at
least two-thirds of the shares of the affected series of Unocal Preferred Stock
and $3.50 Preferred Stock outstanding at the time (voting separately as a class
with all other affected series of preferred stock ranking on a parity with the
affected series of Unocal Preferred Stock and $3.50 Preferred Stock), (i)
authorize, create or issue, or increase the authorized amount of, any class or
series of capital stock ranking prior to the affected series of Unocal Preferred
Stock and $3.50 Preferred Stock as to dividends or upon liquidation; or (ii)
amend, alter or repeal the provisions of Unocal's Certificate of Incorporation,
whether by merger, consolidation or otherwise, so as to materially and adversely
affect any right, preference, privilege or voting power of the affected series
of Unocal Preferred Stock or $3.50 Preferred Stock or the holders thereof;
provided, however, that any increase in the amount of the authorized Common
Stock or authorized Preferred Stock or the creation and issuance of other series
of capital stock ranking on a parity with or junior to the affected series of
Unocal Preferred Stock or $3.50 Preferred Stock as to dividends and upon
liquidation shall not be deemed to materially and adversely affect such rights,
preferences, privileges or voting powers.
 
DESCRIPTION OF OUTSTANDING PREFERRED STOCK
 
    As of the date of this Prospectus Unocal has issued and outstanding
10,250,000 shares of the $3.50 Preferred Stock, which is senior to the Unocal
Common Stock as to payment of dividends and distribution of assets on
liquidation, dissolution or winding up of Unocal.
 
    Holders of $3.50 Preferred Stock are entitled to receive, when and as
declared by Unocal's Board of Directors, out of Unocal's funds legally available
for payment, quarterly cash dividends at an annual rate of $3.50 per share
payable in arrears. Dividends are cumulative.
 
    In the event of any voluntary or involuntary dissolution, liquidation or
winding up of Unocal, the holders of $3.50 Preferred Stock will be entitled to
receive and to be paid out of Unocal's assets available for distribution to its
stockholders, before any payment or distribution is made to holders of Unocal
Common Stock or any other class of capital stock of Unocal ranking junior to the
$3.50 Preferred Stock upon liquidation, a liquidation preference in the amount
of $50 per share of the Unocal Preferred Stock plus accrued and unpaid
dividends.
 
    The $3.50 Preferred Stock is not subject to any mandatory redemption,
sinking fund or other similar provisions. At any time on or after July 15, 1996,
the $3.50 Preferred Stock is redeemable in whole or in part, at Unocal's option,
at redemption prices declining from 104.2% at July 15, 1996 to 100% at July 15,
2002.
 
    The $3.50 Preferred Stock is convertible at the option of the holder at any
time, unless previously redeemed, into Unocal Common Stock at the rate of 1.6260
shares of Unocal Common Stock for each share of $3.50 Preferred Stock
(equivalent to a conversion price of $30.75 per share of Unocal Common Stock).
The conversion price is subject to adjustment in certain events, including a
Non-Stock Fundamental Change or a Common Stock Fundamental Change (as such terms
are defined in the Certificate of Designations relating thereto).
 
    Holders of $3.50 Preferred Stock have no right to require redemption of the
$3.50 Preferred Stock.
 
                                       22
<PAGE>
                        DESCRIPTION OF THE COMMON STOCK
 
    Unocal's Board of Directors is authorized to issue a maximum of 750,000,000
shares of Unocal Common Stock, $1.00 par value per share, under Unocal's
Certificate of Incorporation. As of December 31, 1994, 244,198,701 shares of
Unocal Common Stock were outstanding and 32,925,005 shares were reserved for
issuance upon the conversion of the $3.50 Preferred Stock and in connection with
Unocal's employee benefit plans, its Directors' Restricted Stock Plan and its
Dividend Reinvestment and Common Stock Purchase Plan.
 
    The following summary of the rights of the Unocal Common Stock does not
purport to be complete and is subject in all respects to the applicable
provisions of the Delaware General Corporation Law and the Certificate of
Incorporation.
 
    DIVIDEND RIGHTS:  Subject to the prior rights, if any, of the holders of
$3.50 Preferred Stock and Unocal Preferred Stock, holders of Unocal Common Stock
are entitled to receive such dividends as are declared by Unocal's Board of
Directors out of funds legally available therefor.
 
    VOTING RIGHTS:  Subject to the rights, if any, of the holders of $3.50
Preferred Stock and Unocal Preferred Stock, all voting rights are vested in the
holders of shares of Unocal Common Stock, each share being entitled to one vote
on all matters presented for a vote (except for those matters for which a
separate class vote is required under Delaware law). The holders of one-third of
the shares entitled to vote constitute a quorum at any meeting of stockholders.
 
    LIQUIDATION RIGHTS:  Subject to the rights, if any, of the holders of $3.50
Preferred Stock and Unocal Preferred Stock, in the event of liquidation of
Unocal, holders of Unocal Common Stock will share pro rata in all assets
distributable to stockholders in respect of shares held by them.
 
    PREEMPTIVE RIGHTS:  Holders of Unocal Common Stock are not entitled to any
preemptive rights to subscribe for any additional securities that may be issued.
 
    NON-CUMULATIVE VOTING:  Holders of shares of Unocal Common Stock have
non-cumulative voting rights, which means that holders of more than 50% of the
shares voting for the election of directors can elect 100% of the directors
standing for election if they choose to do so, and, in such event, the holders
of the remaining less than 50% of the shares voting for the election of
directors will not be able to elect any person or persons to the Board of
Directors of Unocal. Unocal's Board of Directors is divided into three classes,
and directors are normally elected for three-year terms. One of the classes is
presented for election at each annual meeting, so that the entire Board of
Directors is never presented for election in any one year.
 
    Chemical Trust Company of California, Los Angeles, California, is the
transfer agent and registrar for the Unocal Common Stock. The Unocal Common
Stock may also be presented for transfer at the office of Chemical Bank, New
York, New York.
 
RIGHTS TO PURCHASE SERIES A PREFERRED STOCK
 
    In January 1990, the Board of Directors of Unocal adopted a stockholder
rights plan (the "Rights Plan") and declared a dividend of one right (a "Right";
collectively, the "Rights") for, and to be attached to, each outstanding share
of Common Stock. The resolutions creating the Rights Plan provide that as long
as the Rights are attached to shares of Common Stock, as provided in the "Rights
Agreement" referred to below, one additional Right will be issued and delivered
with each share of Common Stock that becomes outstanding after February 12,
1990. Each Right entitles the holder thereof to purchase one one-hundredth of a
share of preferred stock designated as the Series A Junior Participating
Cumulative Preferred Stock ("Series A Preferred Stock"). The Rights will expire
on January 29, 2000, unless redeemed earlier, and will not be exercisable or
transferable separately from the shares of Common Stock until the close of
business on the Distribution Date, which will occur on the earlier of (i) the
tenth day following a public
 
                                       23
<PAGE>
announcement that a person or group of affiliated or associated persons (a "15%
Stockholder") has acquired, or obtained the right to acquire, beneficial
ownership of 15% or more of the outstanding Common Stock or (ii) the date of the
commencement or the announcement of an intention to make a tender or exchange
offer that would cause any person or group to become a 15% Stockholder.
 
    Pursuant to the Rights Plan, 3,000,000 shares of Series A Preferred Stock
have been designated and reserved for issuance upon exercise of the Rights. An
additional number of shares of Series A Preferred Stock equal to one
one-hundredth of the number of shares of Unocal Common Stock will be reserved
for issuance in connection with an issuance of Preferred Stock or Unocal Common
Stock, whether issued directly, upon exercise of Equity Warrants or upon
conversion of Preferred Stock or Debt Securities.
 
    A description of the Rights and the Series A Preferred Stock is set forth in
the Rights Agreement, dated January 29, 1990, between Unocal and Chemical Trust
Company of California, as Rights Agent, which is included as exhibit to the
Registration Statement of which this Prospectus is a part.
 
CERTAIN PROVISIONS OF THE CERTIFICATE OF INCORPORATION AND BYLAWS OF UNOCAL
 
    The Certificate of Incorporation and Bylaws of Unocal contain certain
provisions which may have the effect of rendering a change of control of Unocal
more difficult. The Certificate of Incorporation provides that the Board of
Directors is divided into three classes, with the directors serving three-year
staggered terms. Special meetings of Unocal's stockholders generally may be
called only by the Board of Directors, and any action required or permitted to
be taken by the stockholders must be taken at an annual or special meeting and
may not be effected by written consent. The vote of 75% of the outstanding stock
of Unocal entitled to vote is required for the stockholders to adopt, amend or
repeal bylaws. Such a 75% vote is also required for approval of a merger or
consolidation of Unocal with, and certain other transactions with, another
corporation which, with its affiliates, owns beneficially more than 10% of the
total voting power of all outstanding shares of Unocal voting stock (a "Related
Corporation"), unless such a transaction was approved by 75% of the directors of
Unocal prior to the Related Corporation becoming such. The Certificate of
Incorporation also require such a 75% vote to repeal or amend any of the
foregoing provisions.
 
    The Bylaws of Unocal require 30 days' advance notice of, and specified
information with respect to, nominations by stockholders of persons for election
as directors and other business to be brought before an annual meeting by a
stockholder.
 
    As set forth above under "Description of the Preferred Stock," the Board of
Directors has the authority, without further stockholder action, to provide for
the issuance of Unocal Preferred Stock and to fix the terms thereof. Provisions
which could render a change of control of Unocal more difficult, such as
extraordinary voting, dividend, redemption or conversion rights, could be
included in such Unocal Preferred Stock.
 
                          DESCRIPTION OF THE WARRANTS
 
    The following description sets forth certain general terms and provisions of
the Debt Warrants and Equity Warrants to which a Prospectus Supplement may
relate. The particular terms of any Debt Warrants and Equity Warrants offered
will be described in the Prospectus Supplement relating to such Debt Warrants or
Equity Warrants.
 
    The following summaries of certain provisions of the Debt Warrants and
Equity Warrants and of one or more separate Warrant Agreements (each a "Warrant
Agreement") between the Company and Unocal and one or more banking institutions
or trust companies, as Warrant Agent (each a "Warrant Agent"), do not purport to
be complete and are subject to and qualified in their entirety by reference to
all provisions
 
                                       24
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of the applicable Warrant Agreement. Forms of Warrant Agreements are filed as
exhibits to the Registration Statement. Each Warrant Agreement will be governed
by, and construed in accordance with, the laws of the State of New York.
 
GENERAL
 
    Debt Warrants and Equity Warrants, evidenced by Warrant Certificates (the
"Warrant Certificates"), may be issued under a Warrant Agreement independently
or together with any Debt Securities, Unocal Preferred Stock or Unocal Common
Stock and may be transferable with or separate from such Securities. If Debt
Warrants to purchase Debt Securities are offered, the applicable Prospectus
Supplement will describe the terms of the Debt Warrants, including the
following: (i) the offering price, if any, including the currency, or currency
unit in which such price will be payable; (ii) the designation, aggregate
principal amount and terms of the Offered Debt Securities with which the Debt
Warrants are issued and the number of Debt Warrants issued with each such
Offered Debt Security; (iii) if applicable, the date on or after which the Debt
Warrants and the related Offered Debt Securities will be separately
transferable; (iv) the designation, aggregate principal amount and terms of Debt
Securities purchasable upon exercise of one Debt Warrant and the price or prices
at which, and the currency, or currency unit in which such principal amount of
Debt Securities may be purchased upon exercise; (v) the date on which the right
to exercise the Debt Warrants commences and the date on which such right
expires; (vi) any United States Federal income tax consequences; (vii) whether
the Debt Warrants represented by the Warrant Certificates will be issued in
registered or bearer form or both; and (viii) any other material terms of the
Debt Warrants. If Equity Warrants are offered, the applicable Prospectus
Supplement will describe the terms of the Equity Warrants, including the
following: (i) the offering price, if any, including the currency or currency
unit in which such price will be payable; (ii) the designation of any series of
Unocal Preferred Stock purchasable upon exercise of the Equity Warrants; (iii)
the number of shares of Unocal Preferred Stock or Unocal Common Stock
purchasable upon exercise of one Equity Warrant, and the price or prices at
which, and the currency, or currency unit in which such shares may be purchased
upon exercise; (iv) the date on which the right to exercise the Equity Warrants
commences and the date on which such right expires; (v) any United States
Federal income tax consequences; (vi) whether the Equity Warrants represented by
the Warrant Certificate will be issued in registered or bearer form or both;
(vii) whether the Equity Warrants or the underlying Unocal Preferred Stock or
Unocal Common Stock will be listed on any national securities exchange; and
(viii) any other material terms of the Equity Warrants. In addition, if any Debt
Warrants or Equity Warrants are sold for any foreign currency or currency units,
the restrictions, elections, tax consequences, specific terms and other
information with respect to such issue will be specified in the applicable
Prospectus Supplement.
 
    Warrant Certificates, if any, may be exchanged for new Warrant Certificates
of different denominations and may (if in registered form) be presented for
registration of transfer at the corporate trust office of the Warrant Agent,
which will be listed in the applicable Prospectus Supplement, or at such other
office as may be set forth therein. Warrantholders do not have any of the rights
of holders of Debt Securities (except to the extent that the consent of
Warrantholders may be required for certain modifications of the terms of the
Indenture under which the series of Offered Debt Securities issuable upon
exercise of the Warrants are to be issued) or Unocal Preferred or Common
stockholders and are not entitled to payments of principal and interest, if any,
on Debt Securities or to dividends or other distributions made with respect to
Unocal Preferred Stock or Unocal Common Stock.
 
EXERCISE OF WARRANTS
 
    Warrants may be exercised by surrendering the Warrant Certificate, if any,
at the corporate trust office or other designated office of the Warrant Agent,
with (i) the form of election to purchase on the reverse side of the Warrant
Certificate, if any, properly completed and executed, and (ii) payment in full
of the exercise price, as set forth in the applicable Prospectus Supplement.
Upon exercise of Warrants, the
 
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Warrant Agent will, as soon as practicable, deliver the Debt Securities, Unocal
Preferred Stock or Unocal Common Stock issuable upon the exercise of the
Warrants in authorized denominations in accordance with the instructions of the
exercising Warrantholder and at the sole cost and risk of such holder. If less
than all of the Warrants evidenced by the Warrant Certificate are exercised, a
new Warrant Certificate will be issued for the remaining amount of unexercised
Warrants, if sufficient time exists prior to the expiration date.
 
                LIMITATIONS ON THE ISSUANCE OF BEARER SECURITIES
 
    In compliance with United States Federal tax laws and regulations, Bearer
Securities may not, in general, be offered or sold during the Restricted Period
(as defined below) to a person within the United States or to, or for the
account or benefit of, a United States person. However, offers or sales can be
made to (i) the United States office of international organizations (as defined
in Section 7701(a)(18) of the Internal Revenue Code of 1986, as amended (the
"Code") and the regulations thereunder), (ii) the United States office of
foreign central banks (as defined in Section 895 of the Code and the regulations
thereunder) and (iii) foreign branches of United States financial institutions
which are purchasing for their own account or for resale, and which have agreed
to comply with the reporting requirements of Section 165(j)(3)(A), (B) or (C) of
the Code and the regulations thereunder. In addition, sales can be made to a
United States person acquiring a Bearer Security through a financial institution
described in clause (iii) of the preceding sentence if certain certification
requirements and other conditions are satisfied. Definitive Bearer Securities
will not be delivered within the United States, or in any event unless the
beneficial owner of the Securities has complied with the certification
requirements to be described in the relevant Prospectus Supplement.
 
    Each underwriter, dealer and agent (or other "distributor" within the
meaning of the regulations under Section 163 of the Code) participating in the
distribution of any Bearer Securities will agree that (i) it will not offer,
sell or deliver Bearer Debt Securities within the United States or to, or for
the account or benefit of, United States persons (other than qualifying
financial institutions) (a) until 40 days after the closing date or (b) at any
time if the obligation is held as part of an unsold allotment or subscription
(the "Restricted Period"), and (ii) it has in effect procedures reasonably
designed to ensure that its employees and agents who are directly engaged in
selling the Bearer Securities are aware of the restrictions described in clause
(i) of this sentence. Bearer Securities will bear a legend on their face and on
any interest coupons that may be detached therefrom or, if the obligation is
evidenced by a book entry, a legend will appear in the book of record in which
the book entry is made substantially to the following effect: "Any United States
person who holds this obligation will be subject to limitations under the United
States income tax laws, including the limitations provided in Section 165(j) and
1287(a) of the Internal Revenue Code." The Code Sections referred to in such
legend provide that a United States person who holds a Bearer Security will not
be allowed to deduct any loss realized on the sale, exchange or redemption of
such Bearer Security and any gain (which might otherwise be characterized as
capital gain) recognized on such sale, exchange or redemption will be treated as
ordinary income. If the Company or Unocal issue Warrants in bearer form, they
will specify in the applicable Prospectus Supplement what, if any, restrictions
or certification requirements will be applicable to the issuance and delivery of
such bearer Warrants.
 
    As used herein, "United States person" means an individual who is a citizen
or resident of the United States, a corporation, partnership or other entity
created or organized in or under the laws of the United States or any political
subdivision thereof, or an estate or trust the income of which is subject to
United States Federal income taxation regardless of its source; and "United
States" means the United States of America (including the States and the
District of Columbia) and its possessions, which include, as of the date hereof,
Puerto Rico, the U.S. Virgin Islands, Guam, American Samoa, Wake Island, and
Northern Mariana Islands.
 
                                       26
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                              PLAN OF DISTRIBUTION
 
    The Company and Unocal may offer and sell the Securities in any of three
ways: (i) directly to investors; (ii) to investors through agents; or (iii)
through underwriters or dealers. The Securities may also be exchanged for
outstanding securities of the Company or Unocal or both and resold by the holder
pursuant to this Prospectus in the over-the-counter market, on the New York
Stock Exchange, through negotiated transactions or otherwise, at market prices
prevailing at the time of sale or at prices otherwise negotiated. The terms of
any such exchange and the method of resale by the holder will be set forth in a
Prospectus Supplement. The applicable Prospectus Supplement with respect to the
Securities will set forth the terms of the offering of the Securities, including
the name or names of any underwriters, the purchase price of the Securities and
the proceeds to the Company or Unocal, as the case may be, from such sale, any
underwriting discounts and other items constituting underwriters' compensation,
any initial public offering price and any discounts or concessions allowed or
reallowed or paid to dealers and any securities exchanges on which such
Securities may be listed.
 
    If underwriters are used in the sale, the Securities will be acquired by the
underwriters for their own account and may be resold from time to time in one or
more transactions, including negotiated transactions, at a fixed public offering
price or at varying prices determined at the time of sale. The Securities may be
either offered to the public through underwriting syndicates represented by
managing underwriters, or directly by one or more underwriters. Unless otherwise
set forth in the applicable Prospectus Supplement, the obligations of the
underwriters to purchase the Securities will be subject to certain conditions
precedent and the underwriters will be obligated to purchase all the Securities
if any are purchased. Any initial public offering price and any discounts or
concessions allowed or reallowed or paid to dealers may be changed from time to
time.
 
    Except for Unocal Common Stock, each issue of Securities sold will be a new
issue of securities with no established trading market. Any underwriters or
agents with respect to an issue of Securities may make a market in such
Securities, but such underwriters or agents will not be obligated to do so and
may discontinue any market making at any time without notice. No assurance can
be given as to the liquidity of any Securities in the secondary market.
 
    If the Securities are issued in exchange for outstanding securities of the
Company or Unocal, the applicable Prospectus Supplement will set forth the terms
of the exchange, the identity of and the terms of sale of the Securities by the
selling security holders.
 
    Securities may be sold directly by the Company or Unocal or through agents
designated by the Company or Unocal from time to time. Any agent involved in the
offer or sale of the Securities in respect of which this Prospectus is delivered
will be named, and any commissions payable by the Company or Unocal to such
agent will be set forth, in the applicable Prospectus Supplement. Unless
otherwise indicated in the applicable Prospectus Supplement, any such agent will
be acting on a best efforts basis for the period of its appointment.
 
    If so indicated in the applicable Prospectus Supplement, the Company or
Unocal will authorize agents, underwriters or dealers to solicit offers by
certain specified institutions to purchase the Securities from the Company at
the public offering price set forth in the applicable Prospectus Supplement
pursuant to delayed delivery contracts providing for payment and delivery on a
specified date in the future. Such contracts will be subject only to those
conditions set forth in the applicable Prospectus Supplement and the applicable
Prospectus Supplement will set forth the commission payable for solicitation of
such contracts.
 
    Agents, selling security holders and underwriters may be entitled under
agreements entered into with the Company and Unocal to indemnification by the
Company and Unocal against certain civil liabilities, including certain
liabilities under the Securities Act of 1933, or to contribution with respect to
payments which the agents, selling security holders or underwriters may be
required to make in respect thereof.
 
                                       27
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Agents, selling security holders and underwriters may be customers of, engage in
transactions with, or perform services for the Company or Unocal in the ordinary
course of business.
 
                                    EXPERTS
 
    The consolidated financial statements and financial statement schedules of
the Company and Unocal as of December 31, 1993 and 1992, and for each of the
three years in the period ended December 31, 1993, included in the 1993 Annual
Reports on Form 10-K of the Company and of Unocal incorporated by reference in
this Prospectus, have been incorporated herein in reliance on the reports of
Coopers & Lybrand, independent accountants, which reports are incorporated by
reference herein, and given on the authority of that firm as experts in
accounting and auditing. Each of such reports includes an explanatory paragraph
with respect to the changes in methods of accounting for income taxes in 1992
and for postretirement benefits other than pensions and for postemployment
benefits in 1993.
 
    The information concerning estimates of proved oil and gas and geothermal
reserves attributable to the Company and Unocal, included in the 1993 Annual
Reports on Form 10-K of the Company and Unocal incorporated by reference in this
Prospectus, has been prepared by the Company's petroleum engineering staff and
certified by John F. Imle, Jr., President of Unocal and the Company and formerly
President of the Energy Resources Division of Unocal and the Company, and has
been incorporated by reference herein in reliance upon the authority of Mr. Imle
as an expert in the field of petroleum engineering. As of December 31, 1994, Mr.
Imle owned 60,296 shares of Unocal Common Stock, which included 7,149 restricted
shares that vest in 1997 and 1998. Mr. Imle also held options to purchase
150,162 shares of Unocal Common Stock at prices ranging from $11.1563 to
$30.0625, with expiration dates ranging from 1996 to 2004. In addition, he held
27,157 performance share units initially awarded to him in 1992 through 1994,
payable four years after the award dates. Each unit is the equivalent of one
share of Unocal Common Stock. The number of units actually paid out at the end
of each four-year term could be between 0% and 200% of the units initially
awarded, depending upon Unocal's total return to stockholders compared to that
of a peer group of companies. However, the value of the units paid out may not
exceed 400% of the value of the units initially awarded. The units are paid out
in a combination of cash and shares of Unocal Common Stock, as determined by the
Compensation Committee of the Board of Directors.
 
                                 LEGAL MATTERS
 
    Legal matters in connection with the issuance and sale of the Securities
offered hereby will be passed upon for the Company and Unocal by Dennis P. R.
Codon, Esq., Vice President and General Counsel of the Company and Unocal, and
for any underwriters, selling security holders or agents by Brobeck, Phleger &
Harrison, Los Angeles, California. As of December 31, 1994, Mr. Codon owned
11,212 shares of Unocal Common Stock, which included 7,002 restricted shares
that vest in 1995 through 1999. He also held options to purchase 30,267 shares
of Unocal Common Stock at prices ranging from $20.5313 to $30.0625, with
expiration dates ranging from 2000 to 2004. In addition, Mr. Codon held 10,679
performance share units awarded to him in 1992 through 1994, payable as
explained in the preceding paragraph. Brobeck, Phleger & Harrison also
represents the Company and Unocal in certain legal matters.
 
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