<PAGE>
                                                                    EXHIBIT 4.13

                                     FORM OF

                    PREFERRED SECURITIES GUARANTEE AGREEMENT

                             UNOCAL CAPITAL TRUST II

                          DATED AS OF
                                      ---------------,-----


<PAGE>

                                TABLE OF CONTENTS

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                                                                                                      Page
                                                                                                      ----

                                    ARTICLE I

                         Definitions and Interpretation
<S>                                                                                                   <C>
Section 1.1.  Definitions and Interpretation.......................................................      1

                                   ARTICLE II

                               Trust Indenture Act

Section 2.1.  Trust Indenture Act; Application.....................................................      4
Section 2.2.  Lists of Holders.....................................................................      4
Section 2.3.  Reports by the Preferred Guarantee Trustee...........................................      5
Section 2.4.  Periodic Reports to Preferred Guarantee Trustee......................................      5
Section 2.5.  Evidence of Compliance with Conditions Precedent.....................................      5
Section 2.6.  Events of Default; Waiver............................................................      5
Section 2.7.  Event of Default; Notice.............................................................      5
Section 2.8.  Conflicting Interests................................................................      6

                                   ARTICLE III

            Powers, Duties and Rights of Preferred Guarantee Trustee

Section 3.1.  Powers and Duties of the Preferred Guarantee Trustee.................................      6
Section 3.2.  Certain Rights of Preferred Guarantee Trustee........................................      7
Section 3.3.  Not Responsible for Recitals or Issuance of Preferred Securities Guarantee...........      9

                                   ARTICLE IV

                           Preferred Guarantee Trustee

Section 4.1.  Preferred Guarantee Trustee Eligibility.............................................       10
Section 4.2.  Appointment Removal and Resignation of Preferred Guarantee Trustee..................       10

                                    ARTICLE V

                         Preferred Securities Guarantee

Section 5.1.  Preferred Securities Guarantee......................................................       11
Section 5.2.  Waiver of Notice and Demand.........................................................       11
Section 5.3.  Obligations Not Affected............................................................       11
Section 5.4.  Rights of Holders...................................................................       12
Section 5.5.  Guarantee of Payment................................................................       12
Section 5.6.  Subrogation.........................................................................       12
Section 5.7.  Independent Obligations.............................................................       13
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<TABLE>
                                   ARTICLE VI

                    Limitation of Transactions; Subordination
<S>                                                                                                      <C>
Section 6.1.  Limitation of Transactions..........................................................       13
Section 6.2.  Subordination.......................................................................       13

                                   ARTICLE VII

                                   Termination

Section 7.1.  Termination.........................................................................       14

                                  ARTICLE VIII

                                 Indemnification

Section 8.1.  Exculpation.........................................................................       14
Section 8.2.  Indemnification.....................................................................       14

                                   ARTICLE IX

                                  Miscellaneous

Section 9.1.  Successors and Assigns..............................................................       15
Section 9.2.  Amendments..........................................................................       15
Section 9.3.  Notices.............................................................................       15
Section 9.4.  Benefit.............................................................................       16
Section 9.5.  Governing Law.......................................................................       16
</TABLE>


                                      -ii-
<PAGE>
                    PREFERRED SECURITIES GUARANTEE AGREEMENT

      This GUARANTEE AGREEMENT (the "Preferred Securities Guarantee") dated as
of __________, ____, is executed and delivered by Unocal Corporation, a Delaware
corporation (the "Guarantor"), and The Bank of New York, as trustee (the
"Preferred Guarantee Trustee"), for the benefit of the Holders (as defined
herein) from time to time of the Preferred Securities (as defined herein) of
Unocal Capital Trust II, a Delaware statutory trust (the "Issuer").

      WHEREAS, pursuant to an Amended and Restated Declaration of Trust (the
"Declaration") dated as of __________, ____, among the trustees of the Issuer
named therein, the Guarantor, as sponsor, and the holders from time to time of
undivided beneficial interests in the assets of the Issuer, the Issuer is
issuing to the Holders as of the date hereof up to _________ preferred
securities, having an aggregate liquidation amount of up to $_______ designated
the "___% Trust [Convertible] Preferred Securities" (the "Preferred
Securities");

      WHEREAS, as an incentive to purchase such Preferred Securities, the
Guarantor desires fully and unconditionally to agree, to the extent set forth in
this Preferred Securities Guarantee, to pay to the Holders of the Preferred
Securities the Guarantee Payments (as defined herein) and to make certain other
payments on the terms and conditions set forth herein; and

      WHEREAS, as of the date hereof, the Guarantor is also executing and
delivering a guarantee agreement (the "Common Securities Guarantee") in
substantially identical terms to this Preferred Securities Guarantee for the
benefit of the holders of the common securities of the Issuer (the "Common
Securities"), except that if an Event of Default (as defined in the Declaration)
has occurred and is continuing, the rights of holders of the Common Securities
to receive payments under the Common Securities Guarantee are subordinated to
the rights of Holders of Preferred Securities to receive Guarantee Payments
under this Preferred Securities Guarantee.

      NOW, THEREFORE, in consideration of the foregoing, the Guarantor executes
and delivers this Preferred Securities Guarantee for the benefit of the Holders
of Preferred Securities.

                                    ARTICLE I
                         Definitions and Interpretation

      Section 1.1. Definitions and Interpretation. In this Preferred Securities
Guarantee, unless the context otherwise requires:

      (a)   capitalized terms used in this Preferred Securities Guarantee but
not defined in the preamble above have the respective meanings assigned to them
in this Section 1.1;

      (b)   terms defined in the Declaration as of the date of execution of this
Preferred Securities Guarantee have the same meaning when used in this Preferred
Securities Guarantee unless otherwise defined in this Preferred Securities
Guarantee;

      (c)   a term defined anywhere in this Preferred Securities Guarantee has
the same meaning throughout;
<PAGE>
      (d)   all references to "the Preferred Securities Guarantee" or "this
Preferred Securities Guarantee" are to this Preferred Securities Guarantee as
modified, supplemented or amended from time to time;

      (e)   all references in this Preferred Securities Guarantee to Articles
and Sections are to Articles and Sections of this Preferred Securities
Guarantee, unless otherwise specified;

      (f)   a term defined in the Trust Indenture Act of 1939, as amended (the
"Trust Indenture Act") has the same meaning when used in this Preferred
Securities Guarantee, unless otherwise defined in this Preferred Securities
Guarantee or unless the context otherwise requires; and

      (g)   a reference to the singular includes the plural and vice versa.

      "Authorized Officer" of a Person means any Person that is authorized to
bind such Person, provided, however, that the Authorized Officer signing a
certificate furnished pursuant to Section 314(a)(4) of the Trust Indenture Act
shall be the principal executive, financial or accounting officer of such
Person.

      "Common Securities" has the meaning specified in the recitals.

      "Common Securities Guarantee" has the meaning specified in the recitals.

      "Corporate Trust Office" means the office of the Preferred Guarantee
Trustee at which the corporate trust business of the Preferred Guarantee Trustee
shall, at any particular time, be principally administered, which office at the
date of execution of this Preferred Securities Guarantee is located at 101
Barclay Street (21 West), New York, New York 10286.

      "Covered Person" means any Holder or beneficial owner of Preferred
Securities.

      "Declaration" has the meaning specified in the recitals.

      "Event of Default" means a default by the Guarantor on any of its payment
or other obligations under this Preferred Securities Guarantee; provided,
however, that, except with respect to a default in respect of any Guarantee
Payment (or a default under Section 6.1 hereof as a result of an event of
default under the Declaration), any such default shall constitute an Event of
Default only if the Guarantor shall have received written notice of such default
and shall not have cured such default within 60 days after receipt of such
notice.

      "Guarantee Payments" means the following payments or distributions,
without duplication, with respect to the Preferred Securities, to the extent not
paid or made by the Issuer: (a) any accumulated and unpaid Distributions (as
defined in the Declaration) that are required to be paid on such Preferred
Securities to the extent the Issuer shall have funds available therefor, (b) the
redemption price (the "Redemption Price"), and all accumulated and unpaid
Distributions to the date of redemption, to the extent the Issuer has funds
available therefor, with respect to any Preferred Securities called for
redemption by the Issuer, and (c) upon a voluntary or involuntary dissolution,
winding-up or termination of the Issuer (other than in connection with [the
conversion of all of the Trust Securities into the Guarantor's common stock or]
the


                                      -2-
<PAGE>
distribution of Debentures (as defined in the Declaration) to the Holders in
exchange for Preferred Securities as provided in the Declaration), the lesser of
(i) the aggregate of the liquidation amount and all accrued and unpaid
Distributions on the Preferred Securities to the date of payment, to the extent
the Issuer shall have funds available therefor and (ii) the amount of assets of
the Issuer remaining available for distribution to Holders in liquidation of the
Issuer (in either case, the "Liquidation Distribution").

      "Guarantor" has the meaning specified in the preamble.

      "Holder" shall mean any holder, as registered on the books and records of
the Issuer, of any Preferred Securities; provided, however, that, in determining
whether the holders of the requisite percentage of Preferred Securities have
given any request, notice, consent or waiver hereunder, "Holder" shall not
include the Guarantor or any Affiliate (as defined in the Declaration) of the
Guarantor.

      "Indemnified Person" means the Preferred Guarantee Trustee, any Affiliate
of the Preferred Guarantee Trustee, or any officers, directors, shareholders,
members, partners, employees, representatives, nominees, custodians or agents of
the Preferred Guarantee Trustee.

      "Indenture" means the Multiple-Series Indenture dated as of September 11,
1996, of the Guarantor, as supplemented by the Second Supplemental Indenture
dated as of _________ ____, between the Guarantor and The Bank of New York, as
trustee, relating to the Debentures.

      "Issuer" has the meaning specified in the preamble.

      "Liquidation Distribution" has the meaning specified in the definition of
"Guarantee Payments."

      "List of Holders" has the meaning specified in Section 2.2.

      "Majority in liquidation amount of the Preferred Securities" means, except
as provided in the terms of the Preferred Securities, or except as provided by
the Trust Indenture Act, a vote by Holder(s), voting separately as a class, of
more than 50% of the liquidation amount (including the stated amount that would
be paid on redemption, liquidation or otherwise, plus accumulated and unpaid
Distributions to the date upon which the voting percentages are determined) of
all outstanding Preferred Securities.

      "1996 Debentures" means the Guarantor's 6-1/4% Convertible Junior
Subordinated Debentures due 2026.

      "1996 Preferred Securities" means Unocal Capital Trust's 6 1/4% Trust
Convertible Preferred Securities.

      "Person" means a legal person, including any individual, corporation,
estate, partnership, joint venture, association, joint stock company, limited
liability company, trust, statutory or business trust, unincorporated
association, or government or any agency or political subdivision thereof, or
any other entity of whatever nature.


                                      -3-
<PAGE>
      "Preferred Guarantee Trustee" means The Bank of New York, until a
Successor Preferred Guarantee Trustee has been appointed and has accepted such
appointment pursuant to the terms of this Preferred Securities Guarantee and
thereafter means each such Successor Preferred Guarantee Trustee.

      "Preferred Securities" has the meaning specified in the recitals.

      "Preferred Securities Guarantee" has the meaning specified in the
preamble.

      "Redemption Price" has the meaning specified in the definition of
"Guarantee Payments."

      "Responsible Officer" means, with respect to the Preferred Guarantee
Trustee, any officer within the Corporate Trust Office of the Preferred
Guarantee Trustee, including any vice president, any assistant vice president,
any assistant secretary, the treasurer, any assistant treasurer or other officer
within the Corporate Trust Office of the Preferred Guarantee Trustee customarily
performing functions similar to those performed by any of the above designated
officers, and also means, with respect to a particular corporate trust matter,
any other officer to whom such matter is referred because of that officer's
knowledge of and familiarity with the particular subject.

      "Successor Preferred Guarantee Trustee" means a successor Preferred
Guarantee Trustee possessing the qualifications to act as Preferred Guarantee
Trustee under Section 4.1.

      "Trust Securities" means, collectively, the Common Securities and the
Preferred Securities.

                                   ARTICLE II
                               Trust Indenture Act

      Section 2.1. Trust Indenture Act; Application. (a) This Preferred
Securities Guarantee is subject to the provisions of the Trust Indenture Act
that are required to be part of this Preferred Securities Guarantee and shall,
to the extent applicable, be governed by such provisions; and

      (b)   if and to the extent that any provision of this Preferred Securities
Guarantee limits, qualifies or conflicts with the duties imposed by Section 310
to 317, inclusive, of the Trust Indenture Act, such imposed duties shall
control.

      Section 2.2. Lists of Holders. (a) The Guarantor shall provide the
Preferred Guarantee Trustee with a list, in such form as the Preferred Guarantee
Trustee may reasonably require, of the names and addresses of the Holders ("List
of Holders") (i) within 1 Business Day after each record date for payment of
Distributions as long as the Preferred Securities remain in book-entry only
form, otherwise within 14 days after each record date for payment of
Distributions, and (ii) at any other time within 30 days of receipt by the
Guarantor of a written request for a List of Holders as of a date no more than
14 days before such List of Holders is given to the Preferred Guarantee Trustee,
provided, that the Guarantor shall not be obligated to provide such List of
Holders at any time the List of Holders does not differ from the most recent
List of Holders


                                       -4-
<PAGE>
given to the Preferred Guarantee Trustee by the Guarantor. The Preferred
Guarantee Trustee may destroy any List of Holders previously given to it on
receipt of a new List of Holders.

      (b)   The Preferred Guarantee Trustee shall comply with its obligations
under Sections 311(a), 311(b) and Section 312(b) of the Trust Indenture Act.

      Section 2.3. Reports by the Preferred Guarantee Trustee. Within 60 days
after May 15 of each year, the Preferred Guarantee Trustee shall provide to the
Holders such reports as are required by Section 313 of the Trust Indenture Act
if any, in the form and in the manner provided by Section 313 of the Trust
Indenture Act. The Preferred Guarantee Trustee shall also comply with the
requirements of Section 313(d) of the Trust Indenture Act.

      Section 2.4. Periodic Reports to Preferred Guarantee Trustee. The
Guarantor shall provide to the Preferred Guarantee Trustee such documents,
reports and information as are required by Section 314 (if any) and the
compliance certificate required by Section 314 of the Trust Indenture Act in the
form, in the manner and at the times required by Section 314 of the Trust
Indenture Act.

      Delivery of such reports, information and documents to the Preferred
Guarantee Trustee is for informational purposes only and the Preferred Guarantee
Trustee's receipt of such shall not constitute constructive notice of any
information contained therein, including the Guarantor's compliance with any of
its covenants hereunder (as to which the Preferred Guarantee Trustee is entitled
to rely exclusively on Officers' Certificates (as defined in the Declaration)).

      Section 2.5. Evidence of Compliance with Conditions Precedent. The
Guarantor shall provide to the Preferred Guarantee Trustee such evidence of
compliance with any conditions precedent, if any, provided for in this Preferred
Securities Guarantee that relate to any of the matters set forth in Section
314(c) of the Trust Indenture Act. Any certificate or opinion required to be
given by an officer pursuant to Section 314(c)(1) may be given in the form of an
Officers' Certificate.

      Section 2.6. Events of Default; Waiver. The Holders of Majority in
liquidation amount of the Preferred Securities may, by vote, on behalf of the
Holders of all of the Preferred Securities, waive any past Event of Default and
its consequences. Upon such waiver, any such Event of Default shall cease to
exist, and any Event of Default arising therefrom shall be deemed to have been
cured, for every purpose of the Preferred Securities Guarantee, but no such
waiver shall extend to any subsequent or other default or Event of Default or
impair any right consequent thereon.

      Section 2.7. Event of Default; Notice. (a) The Preferred Guarantee Trustee
shall, within 90 days after the occurrence of an Event of Default (or an event
which, with the passage of time or the giving of notice or both, would
constitute an Event of Default), transmit by mail, first class postage prepaid,
to the Holders, notices of all Events of Default or defaults actually known to a
Responsible Officer of the Preferred Guarantee Trustee, unless such defaults
have been cured before the giving of such notice, provided, that, the Preferred
Guarantee Trustee shall be protected in withholding such notice if and so long
as a Responsible Officer of the Preferred


                                      -5-
<PAGE>
Guarantee Trustee in good faith determines that the withholding of such notice
is in the interests of the Holders.

      (b)   The Preferred Guarantee Trustee shall not be deemed to have
knowledge of any Event of Default unless the Preferred Guarantee Trustee shall
have received written notice thereof, or a Responsible Officer of the Preferred
Guarantee Trustee charged with the administration of the Declaration shall have
obtained actual knowledge thereof.

      Section 2.8. Conflicting Interests. The Declaration shall be deemed to be
specifically described in this Preferred Securities Guarantee for the purposes
of clause (i) of the first proviso contained in Section 310(b) of the Trust
Indenture Act.

                                   ARTICLE III
                          Powers, Duties and Rights of
                           Preferred Guarantee Trustee

      Section 3.1. Powers and Duties of the Preferred Guarantee Trustee. (a)
This Preferred Securities Guarantee shall be held by the Preferred Guarantee
Trustee for the benefit of the Holders, and the Preferred Guarantee Trustee
shall not transfer this Preferred Securities Guarantee to any Person except a
Holder exercising his or her rights pursuant to Section 5.4(b) or to a Successor
Preferred Guarantee Trustee upon acceptance by such Successor Preferred
Guarantee Trustee of its appointment to act as Successor Preferred Guarantee
Trustee. The right, title and interest of the Preferred Guarantee Trustee shall
automatically vest in any Successor Preferred Guarantee Trustee, and such
vesting and succession of title shall be effective whether or not conveyance
documents have been executed and delivered pursuant to the appointment of such
Successor Preferred Guarantee Trustee.

      (b)   If an Event of Default actually known to a Responsible Officer of
the Preferred Guarantee Trustee has occurred and is continuing, the Preferred
Guarantee Trustee shall enforce this Preferred Securities Guarantee for the
benefit of the Holders.

      (c)   The Preferred Guarantee Trustee, before the occurrence of any Event
of Default and after the curing of all Events of Default that may have occurred,
shall undertake to perform only such duties as are specifically set forth in
this Preferred Securities Guarantee, and no implied covenants shall be read into
this Preferred Securities Guarantee against the Preferred Guarantee Trustee. In
case an Event of Default has occurred (that has not been cured or waived
pursuant to Section 2.6) and is actually known to a Responsible Officer of the
Preferred Guarantee Trustee, the Preferred Guarantee Trustee shall exercise such
of the rights and powers vested in it by this Preferred Securities Guarantee,
and shall use the same degree of care and skill in its exercise thereof as a
prudent person would exercise or use under the circumstances in the conduct of
his or her own affairs.

      (d)   No provision of this Preferred Securities Guarantee shall be
construed to relieve the Preferred Guarantee Trustee from liability for its own
negligent action, its own negligent failure to act, or its own willful
misconduct, except that:

            (i)   prior to the occurrence of any Event of Default and after the
      curing or waiving of all such Events of Default that may have occurred:


                                      -6-
<PAGE>
                  (A)   the duties and obligations of the Preferred Guarantee
            Trustee shall be determined solely by the express provisions of this
            Preferred Securities Guarantee, and the Preferred Guarantee Trustee
            shall not be liable except for the performance of such duties and
            obligations as are specifically set forth in this Preferred
            Securities Guarantee, and no implied covenants or obligations shall
            be read in this Preferred Securities Guarantee against the Preferred
            Guarantee Trustee; and

                  (B)   in the absence of bad faith on the part of the Preferred
            Guarantee Trustee, the Preferred Guarantee Trustee may conclusively
            rely, as to the truth of the statements and correctness of the
            opinions expressed therein, upon any certificates or opinions
            furnished to the Preferred Guaranteed Trustee and conforming to the
            requirements of this Preferred Securities Guarantee; but in the case
            of any such certificates or opinions that by any provision hereof
            are specifically required to be furnished to the Preferred Guarantee
            Trustee, the Preferred Guarantee Trustee shall be under a duty to
            examine the same to determine whether or not they conform to the
            requirements of this Preferred Securities Guarantee;

            (ii)  the Preferred Guarantee Trustee shall not be liable for any
      error of judgment made in good faith by a Responsible Officer of the
      Preferred Guarantee Trustee, unless it shall be proved that the Preferred
      Guarantee Trustee was negligent in ascertaining the pertinent facts upon
      which such judgment was made;

            (iii) the Preferred Guarantee Trustee shall not be liable with
      respect to any action taken or omitted to be taken by it in good faith in
      accordance with the direction of the Holders of not less than a Majority
      in liquidation amount of the Preferred Securities relating to the time,
      method and place of conducting any proceeding for any remedy available to
      the Preferred Guarantee Trustee, or exercising any trust or power
      conferred upon the Preferred Guarantee Trustee under this Preferred
      Securities Guarantee; and

            (iv)  no provision of this Preferred Securities Guarantee shall
      require the Preferred Guarantee Trustee to expend or risk its own funds or
      otherwise incur personal financial liability in the performance of any of
      its duties or in the exercise of any of its duties or in the exercise of
      any of its rights or powers, if the Preferred Guarantee Trustee shall have
      reasonable grounds for believing that the repayment of such funds or
      liability is not reasonably assured to it under the terms of this
      Preferred Securities Guarantee or indemnity, reasonably satisfactory to
      the Preferred Guarantee Trustee, against such risk or liability is not
      reasonably assured to it.

      Section 3.2. Certain Rights of Preferred Guarantee Trustee. (a) Subject to
the provisions of Section 3.1:

            (i)   The Preferred Guarantee Trustee may conclusively rely, and
      shall be fully protected in acting or refraining from acting upon, any
      resolution, certificate, statement, instrument, opinion, report, notice,
      request, direction, consent, order, bond, debenture,


                                      -7-
<PAGE>
      note, other evidence of indebtedness or other paper or document or
      document believed by it to be genuine and to have been signed, sent or
      presented by the proper party or parties.

            (ii)  Any direction or act of the Guarantor contemplated by this
      Preferred Securities Guarantee shall be sufficiently evidenced by an
      Officers' Certificate.

            (iii) Whenever, in the administration of this Preferred Securities
      Guarantee, the Preferred Guarantee Trustee shall deem it desirable that a
      matter be proved or established before taking, suffering or omitting any
      action hereunder, the Preferred Guarantee Trustee (unless other evidence
      is herein specifically prescribed) may, in the absence of bad faith on its
      part, request and conclusively rely upon an Officers Certificate which,
      upon receipt of such request, shall be promptly delivered by the
      Guarantor.

            (iv)  The Preferred Guarantee Trustee shall have no duty to see to
      any recording, filing or registration of any instrument (or any
      rerecording, refiling or reregistration thereof).

            (v)   The Preferred Guarantee Trustee may consult with counsel of
      its selection, and the advice or opinion of such counsel with respect to
      legal matters shall be full and complete authorization and protection in
      respect of any action taken, suffered or omitted by it hereunder in good
      faith and in accordance with such advice or opinion. Such counsel may be
      counsel to the Guarantor or any of its Affiliates and may include any of
      its employees. The Preferred Guarantee Trustee shall have the right at any
      time to seek instructions concerning the administration of this Preferred
      Securities Guarantee from any court of competent jurisdiction.

            (vi)  The Preferred Guarantee Trustee shall be under no obligation
      to exercise any of the rights or powers vested in it by this Preferred
      Securities Guarantee at the request or direction of any Holder, unless
      such Holder shall have provided to the Preferred Guarantee Trustee such
      security and indemnity, reasonably satisfactory to the Preferred Guarantee
      Trustee, against the costs, expenses (including attorneys fees and
      expenses and the expenses of the Preferred Guarantee Trustee's agents,
      nominees or custodians) and liabilities that might be incurred by it in
      complying with such request or direction, including such reasonable
      advances as may be requested by the Preferred Guarantee Trustee; provided
      that, nothing contained in this Section 3.2(a)(vi) shall be taken to
      relieve the Preferred Guarantee Trustee, upon the occurrence of an Event
      of Default, of its obligations to exercise the rights and powers vested in
      it by this Preferred Securities Guarantee.

            (vii) The Preferred Guarantee Trustee shall not be bound to make any
      investigation into the facts or matters stated in any resolution,
      certificate, statement, instrument, opinion, report, notice, request,
      direction, consent, order, bond, debenture, note, other evidence of
      indebtedness or other paper or document, but the Preferred Guarantee
      Trustee, in its discretion, may make such further inquiry or investigation
      into such facts or matters as it may see fit.


                                      -8-
<PAGE>
            (viii) The Preferred Guarantee Trustee may execute any of the trusts
      or powers hereunder or perform any duties hereunder either directly or by
      or through agents, nominees, custodians or attorneys, and the Preferred
      Guarantee Trustee shall not be responsible for any misconduct or
      negligence on the part of any agent or attorney appointed with due care by
      it hereunder.

            (ix)  Any action taken by the Preferred Guarantee Trustee or its
      agents hereunder shall bind the Holders and the signature of the Preferred
      Guarantee Trustee or its agents alone shall be sufficient and effective to
      perform any such action. No third party shall be required to inquire as to
      the authority of the Preferred Guarantee Trustee to so act or as to its
      compliance with any of the terms and provisions of this Preferred
      Securities Guarantee, both of which shall be conclusively evidenced by the
      Preferred Guarantee Trustee's or its agent's taking such action.

            (x)   Whenever in the administration of this Preferred Securities
      Guarantee the Preferred Guarantee Trustee shall deem it desirable to
      receive instructions with respect to enforcing any remedy or right or
      taking any other action hereunder, the Preferred Guarantee Trustee (i) may
      request instructions from the Holders of a Majority in liquidation amount
      of the Preferred Securities, (ii) may refrain from enforcing such remedy
      or right or taking such other action until such instructions are received,
      and (iii) shall be protected in conclusively relying on or acting in
      accordance with such instructions.

            (xi)  The Preferred Guarantee Trustee may execute any of the trusts
      or powers hereunder or perform any duties hereunder either directly or by
      or through agents or attorneys, and the Preferred Guarantee Trustee shall
      not be responsible for any misconduct or negligence on the part of any
      agent or attorney appointed with due care by it hereunder.

            (xii) The Preferred Guarantee Trustee shall not be liable for any
      action taken, suffered, or omitted to be taken by it in good faith and
      reasonably believed by it to be authorized or within the discretion or
      rights or powers conferred upon it by this Preferred Securities Guarantee.

      (b)   No provision of this Preferred Securities Guarantee shall be deemed
to impose any duty or obligation on the Preferred Guarantee Trustee to perform
any act or acts or exercise any right, power, duty or obligation conferred or
imposed on it in any jurisdiction in which it shall be illegal, or in which the
Preferred Guarantee Trustee shall be unqualified or incompetent in accordance
with applicable law, to perform any such act or acts or to exercise any such
right, power, duty or obligation. No permissive power or authority available to
the Preferred Guarantee Trustee shall be construed to be a duty.

      Section 3.3. Not Responsible for Recitals or Issuance of Preferred
Securities Guarantee. The recitals contained in this Preferred Securities
Guarantee shall be taken as the statements of the Guarantor, and the Preferred
Guarantee Trustee does not assume any responsibility for their correctness. The
Preferred Guarantee Trustee makes no representation as to the validity or
sufficiency of this Preferred Securities Guarantee.


                                      -9-
<PAGE>
                                   ARTICLE IV
                           Preferred Guarantee Trustee

      Section 4.1. Preferred Guarantee Trustee Eligibility. (a) There shall at
all times be a Preferred Guarantee Trustee which shall:

            (i)   not be an Affiliate of the Guarantor; and

            (ii)  be a corporation organized and doing business under the laws
      of the United States of America or any State or Territory thereof or of
      the District of Columbia, or a corporation or Person permitted by the
      Securities and Exchange Commission to act as an institutional trustee
      under the Trust Indenture Act, authorized under such laws to exercise
      corporate trust powers, having a combined capital and surplus of at least
      50 million U.S. dollars ($50,000,000), and subject to supervision or
      examination by federal, state, territorial or District of Columbia
      authority. If such corporation publishes reports of condition at least
      annually, pursuant to law or to the requirements of the supervising or
      examining authority referred to above, then, for the purposes of this
      Section 4.1(a)(ii), the combined capital and surplus of such corporation
      shall be deemed to be its combined capital and surplus as set forth in its
      most recent report of condition so published.

      (b)   If at any time the Preferred Guarantee Trustee shall cease to be
eligible to so act under Section 4.1(a), the Preferred Guarantee Trustee shall
immediately resign in the manner and with the effect set out in Section 4.2(c).

      (c)   If the Preferred Guarantee Trustee has or shall acquire any
"conflicting interest" within the meaning of Section 310(b) of the Trust
Indenture Act, the Preferred Guarantee Trustee and Guarantor shall in all
respects comply with the provisions of Section 310(b) of the Trust Indenture
Act.

      Subject to the foregoing, the Preferred Guarantee Trustee, in its
individual or any other capacity, may become the owner or pledgee of Preferred
Securities and may otherwise deal with the Guarantor with the same rights it
would have if it were not the Preferred Guarantee Trustee.

      Section 4.2. Appointment Removal and Resignation of Preferred Guarantee
Trustee. (a) Subject to Section 4.2(b), the Preferred Guarantee Trustee may be
appointed or removed without cause at any time by the Guarantor.

      (b)   The Preferred Guarantee Trustee shall not be removed in accordance
with Section 4.2(a) until a Successor Preferred Guarantee Trustee has been
appointed and has accepted such appointment by written instrument executed by
such Successor Preferred Guarantee Trustee and delivered to the Guarantor.

      (c)   The Preferred Guarantee Trustee appointed to office shall hold
office until a Successor Preferred Guarantee Trustee shall have been appointed
or until its removal or resignation. The Preferred Guarantee Trustee may resign
from office (without need for prior or subsequent accounting) by an instrument
in writing executed by the Preferred Guarantee Trustee and delivered to the
Guarantor, which resignation shall not take effect until a Successor Preferred
Guarantee Trustee has been appointed and has accepted such appointment by
instrument in


                                      -10-
<PAGE>
writing executed by such Successor Preferred Guarantee Trustee and delivered to
the Guarantor and the resigning Preferred Guarantee Trustee.

      (d)   If no Successor Preferred Guarantee Trustee shall have been
appointed and accepted appointment as provided in this Section 4.2 within 60
days after delivery of an instrument of resignation or removal, the Preferred
Guarantee Trustee resigning or being removed may petition any court of competent
jurisdiction for appointment of a Successor Preferred Guarantee Trustee. Such
court may thereupon, after prescribing such notice, if any, as it may deem
proper, appoint a Successor Preferred Guarantee Trustee.

      (e)   No Preferred Guarantee Trustee shall be liable for the acts or
omissions to act of any Successor Preferred Guarantee Trustee.

      (f)   Upon termination of this Preferred Securities Guarantee or removal
or resignation of the Preferred Guarantee Trustee pursuant to this Section 4.2,
the Guarantor shall pay to the Preferred Guarantee Trustee all amounts owed to
the Preferred Guarantee Trustee by the Guarantor to the date of such
termination, removal or resignation.

                                    ARTICLE V
                         Preferred Securities Guarantee

      Section 5.1. Preferred Securities Guarantee. The Guarantor fully and
unconditionally agrees to pay in full to the Holders the Guarantee Payments
(without duplication of amounts theretofore paid by the Issuer), as and when
due, regardless of any defense, right of set-off or counterclaim that the Issuer
may have or assert. The Guarantor's obligation to make a Guarantee Payment may
be satisfied by direct payment of the required amounts by the Guarantor to the
Holders or by causing the Issuer to pay such amounts to the Holders.

      Section 5.2. Waiver of Notice and Demand. The Guarantor hereby waives
notice of acceptance of this Preferred Securities Guarantee and of any liability
to which it applies or may apply, presentment, demand for payment, any right to
require a proceeding first against the Issuer or any other Person before
proceeding against the Guarantor, protest, notice of nonpayment, notice of
dishonor, notice of redemption and all other notices and demands.
Notwithstanding anything to the contrary herein, the Guarantor retains all of
its rights under the Indenture to extend the interest payment period on the
Debentures and the Guarantor shall not be obligated hereunder to make any
Guarantee Payment during any Extension Period (as defined in the Declaration)
with respect to Distributions on the Preferred Securities.

      Section 5.3. Obligations Not Affected. The obligations, covenants,
agreements and duties of the Guarantor under this Preferred Securities Guarantee
shall in no way be affected or impaired by reason of the happening from time to
time of any of the following:

      (a)   the release or waiver, by operation of law or otherwise, of the
performance or observance by the Issuer of any express or implied agreement,
covenant, term or condition relating to the Preferred Securities to be performed
or observed by the Issuer;

      (b)   the extension of time for the payment by the Issuer of all or any
portion of the Distributions, Redemption Price, Liquidation Distribution or any
other sums payable under the


                                      -11-
<PAGE>
terms of the Preferred Securities or the extension of time for the performance
of any other obligation under, arising out of, or in connection with, the
Preferred Securities (other than an extension of time for payment of
Distributions, Redemption Price, Liquidation Distribution or other sum payable
that results from the extension of any interest payment period on the
Debentures);

      (c)   any failure, omission, delay or lack of diligence on the part of the
Holders to enforce, assert or exercise any right, privilege, power or remedy
conferred on the Holders pursuant to the terms of the Preferred Securities, or
any action on the part of the Issuer granting indulgence or extension of any
kind;

      (d)   the voluntary or involuntary liquidation, dissolution, sale of any
collateral, receivership, insolvency, bankruptcy, assignment for the benefit of
creditors, reorganization, arrangement, composition or readjustment of debt of,
or other similar proceedings affecting, the Issuer or any of the assets of the
Issuer;

      (e)   any invalidity of, or defect or deficiency in, the Preferred
Securities;

      (f)   the settlement or compromise of any obligation guaranteed hereby or
hereby incurred; or

      (g)   any other circumstance whatsoever that might otherwise constitute a
legal or equitable discharge or defense of a guarantor, it being the intent of
this Section 5.3 that the obligations of the Guarantor hereunder shall be full
and unconditional under any and all circumstances.

      There shall be no obligation of the Holders to give notice to, or obtain
consent of, the Guarantor with respect to the happening of any of the foregoing.

      Section 5.4. Rights of Holders. (a) Subject to Section 3.2(a)(vi), the
Holders of a Majority in liquidation amount of the Preferred Securities have the
right to direct the time, method and place of conducting of any proceeding for
any remedy available to the Preferred Guarantor Trustee in respect of this
Preferred Securities Guarantee or exercising any trust or power conferred upon
the Preferred Guarantee Trustee under this Preferred Securities Guarantee.

      (b)   If the Preferred Guarantee Trustee fails to enforce such Preferred
Securities Guarantee as provided in Sections 3.1 and 5.4(a), any Holder of
Preferred Securities may institute a legal proceeding directly against the
Guarantor to enforce the Preferred Guarantee Trustee's rights under this
Preferred Securities Guarantee, without first instituting a legal proceeding
against the Issuer, the Preferred Guarantee Trustee or any other Person. The
Guarantor waives any right or remedy to require that any action be brought first
against the Issuer or any other Person before proceeding directly against the
Guarantor.

      Section 5.5. Guarantee of Payment. This Preferred Securities Guarantee
creates a guarantee of payment and not of collection.

      Section 5.6. Subrogation. The Guarantor shall be subrogated to all (if
any) rights of the Holders of Preferred Securities against the Issuer in respect
of any amounts paid to such


                                      -12-
<PAGE>
Holders by the Guarantor under this Preferred Securities Guarantee; provided,
however, that the Guarantor shall not (except to the extent required by
mandatory provisions of law) be entitled to enforce or exercise any right that
it may acquire by way of subrogation or any indemnity, reimbursement or other
agreement, in all cases as a result of payment under this Preferred Securities
Guarantee, if, at the time of any such payment, any amounts are due and unpaid
under this Preferred Securities Guarantee. If any amount shall be paid to the
Guarantor in violation of the preceding sentence, the Guarantor agrees to hold
such amount in trust for the Holders and to pay over such amount to the Holders.

      Section 5.7. Independent Obligations. The Guarantor acknowledges that its
obligations hereunder are independent of the obligations of the Issuer with
respect to the Preferred Securities, and that the Guarantor shall be liable as
principal and as debtor hereunder to make Guarantee Payments pursuant to the
terms of this Preferred Securities Guarantee notwithstanding the occurrence of
any event referred to in subsections (a) through (g), inclusive, of Section 5.3
hereof.

                                   ARTICLE VI
                    Limitation of Transactions; Subordination

      Section 6.1. Limitation of Transactions. So long as any Preferred
Securities remain outstanding, if there shall have occurred an Event of Default
or an event of default under the Declaration, then (a) the Guarantor shall not
declare or pay any dividend on, make any distributions with respect to, or
redeem, purchase, acquire or make a liquidation payment with respect to, any of
its capital stock (other than (i) purchases or acquisitions of shares of capital
stock in connection with any employee benefit plan or program, director benefit
plan or program, dividend reinvestment, stock repurchase, or other similar plans
available to stockholders of the Guarantor, or any option, warrant, right, or
exercisable, exchangeable, or convertible security outstanding as of
____________, (ii) as a result of a reclassification of the Guarantor's capital
stock or the exchange or conversion of one class or series of the Guarantor's
capital stock for another class or series of the Guarantor's capital stock or
the capital securities of a subsidiary (including a trust such as the Issuer),
(iii) any declaration of a dividend in connection with the implementation of a
shareholder's rights plan, or the issuance of shares under any such plan, or the
redemption or repurchase of any rights issued thereunder, or (iv) the purchase
of fractional interests in shares of the Guarantor's capital stock pursuant to
the conversion or exchange provisions of such capital stock of the Guarantor or
the security being converted or exchanged), and (b) the Guarantor shall not make
any payment of interest, principal or premium, if any, on or repay, repurchase
or redeem, or make any guarantee payments (other than pursuant to this Preferred
Securities Guarantee, if otherwise permitted under the Guarantor's preferred
securities guarantee of the 1996 Preferred Securities , or pursuant to such
guarantee of the 1996 Preferred Securities) with respect to, any debt securities
issued by the Guarantor which rank pari passu with or junior in right of payment
to the Debentures (other than the 1996 Debentures).

      Section 6.2. Subordination. This Preferred Securities Guarantee will
constitute an unsecured obligation of the Guarantor and will rank (i)
subordinate and junior in right of payment to all other liabilities of the
Guarantor, except those made pari passu or subordinate by their terms, (ii) pari
passu with the most senior preferred or preference stock now or hereafter issued
by the Guarantor and with any guarantee now or hereafter entered into by the
Guarantor in


                                      -13-
<PAGE>
respect of such stock or any preferred or preference stock of any Affiliate of
the Guarantor, and (iii) senior to the Guarantor's common stock.

      If an Event of Default (as defined in the Declaration) has occurred and is
continuing, the rights of holders of the Common Securities to receive payments
under the Common Securities Guarantee are subordinated to the rights of Holders
of Preferred Securities to receive Guarantee Payments.

                                   ARTICLE VII
                                   Termination

      Section 7.1. Termination. This Preferred Securities Guarantee shall
terminate upon (i) full payment of the Redemption Price of all Preferred
Securities, (ii) [the distribution of the Guarantor's common stock to all of the
Holders in respect of the conversion of all the Preferred Securities into the
Guarantor's common stock or upon] the distribution of the Debentures held by the
Issuer to the Holders of all of the Preferred Securities or (iii) full payment
of the amounts payable in accordance with the Declaration upon liquidation of
the Issuer. Notwithstanding the foregoing, this Preferred Securities Guarantee
will continue to be effective or will be restated, as the case may be, if at any
time any Holder must restore payment of any sums paid under the Preferred
Securities or under this Preferred Securities Guarantee.

                                  ARTICLE VIII
                                 Indemnification

      Section 8.1. Exculpation. (a) No Indemnified Person shall be liable,
responsible or accountable in damages or otherwise to the Guarantor or any
Covered Person for any loss, damage, liability, expense or claim incurred by
reason of any act or omission performed or omitted by such Indemnified Person in
good faith in accordance with this Preferred Securities Guarantee and in a
manner that such Indemnified Person reasonably believed to be within the scope
of the authority conferred on such Indemnified Person by this Preferred
Securities Guarantee or by law, except that an Indemnified Person shall be
liable for any such loss, damage or claim incurred by reason of such Indemnified
Person's negligence or willful misconduct with respect to such acts or
omissions.

      (b)   An Indemnified Person shall be fully protected in relying in good
faith upon the records of the Guarantor and upon such information, opinions,
reports or statements presented to the Guarantor by any Person as to matters the
Indemnified Person reasonably believes are within such other Person's
professional or expert competence and who has been selected with reasonable care
by or on behalf of the Guarantor, including information, opinions, reports or
statements as to the value and amount of the assets, liabilities, profits,
losses, or any other facts pertinent to the existence and amount of assets from
which Distribution to Holders might properly be paid.

      Section 8.2. Indemnification. The Guarantor agrees to indemnify each
Indemnified Person for, and to hold each Indemnified Person harmless against,
any loss, liability or expense incurred without negligence or bad faith on its
part, arising out of or in connection with the acceptance or administration of
the trust or trusts hereunder, including the costs and expenses


                                      -14-
<PAGE>
(including reasonable legal fees and expenses) of defending itself against, or
investigating, any claim or liability in connection with the exercise or
performance of any of its powers or duties hereunder. The obligation to
indemnify as set forth in this Section 8.2 shall survive the termination of this
Preferred Securities Guarantee.

      When the Preferred Guarantee Trust incurs expenses or renders services in
connection with an Event of Default with respect to the Preferred Securities
specified in Section 6.01(e) or 6.01(f) of the Indenture of the Guarantor, the
expenses (including the reasonable charges and expenses of its counsel) and the
compensation for services are intended to constitute expenses of administration
under any applicable federal or state bankruptcy, insolvency or other similar
law.

                                   ARTICLE IX
                                  Miscellaneous

      Section 9.1. Successors and Assigns. All guarantees and agreements
contained in this Preferred Securities Guarantee shall bind the successors,
assigns, receivers, trustees and representatives of the Guarantor and shall
inure to the benefit of the Holders of the Preferred Securities then
outstanding.

      Section 9.2. Amendments. Except with respect to any changes that do not
adversely affect the rights of Holders in any material respect (in which case no
consent of Holders will be required), this Preferred Securities Guarantee may be
amended only with the prior approval of the Holders of at least a Majority in
liquidation amount (including the stated amount that would be paid on
redemption, liquidation or otherwise, plus accumulated and unpaid Distributions
to the date upon which the voting percentages are determined) of the Preferred
Securities. The provisions of Section 12.2 of the Declaration with respect to
meetings of Holders apply to the giving of such approval.

      Section 9.3. Notices. All notices provided for in this Preferred
Securities Guarantee shall be in writing, duly signed by the party giving such
notice, and shall be delivered by hand, telecopied or mailed by first class
mail, as follows:

      (a)   If given to the Preferred Guarantee Trustee, at the Preferred
Guarantee Trustee's mailing address set forth below (or such other address as
the Preferred Guarantee Trustee may give notice of to the Holders of the
Preferred Securities):

                  The Bank of New York
                  101 Barclay Street (21 West)
                  New York, New York  10286
                  Attn:  Corporate Trust Trustee Administration
                  Fax No.:  _________________________

      (b)   If given to the Guarantor, at the Guarantor's mailing address set
forth below (or such other address as the Guarantor may give notice of to the
Holders of the Preferred Securities):

                  Unocal Corporation
                  2141 Rosecrans Avenue


                                      -15-
<PAGE>
                  Suite 4000
                  El Segundo, California 90245
                  Attn:  Corporate Secretary
                  Fax No.:  (310) 726-7875

      (c)   If given to any Holder of Preferred Securities, at the address set
forth on the books and records of the Issuer.

      All such notices shall be deemed to have been given when received in
person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid, except that if a notice or other document is refused delivery
or cannot be delivered because of a changed address of which no notice was
given, such notice or other document shall be deemed to have been delivered on
the date of such refusal or inability to deliver.

      Section 9.4. Benefit. This Preferred Securities Guarantee is solely for
the benefit of the Holders of the Preferred Securities and, subject to Section
3.1(a), is not separately transferable from the Preferred Securities.

      Section 9.5. Governing Law. THIS PREFERRED SECURITIES GUARANTEE SHALL BE
GOVERNED BY, AND CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE
STATE OF NEW YORK, WITHOUT REGARD TO CONFLICT OF LAWS PRINCIPLES THEREOF.

      THIS PREFERRED SECURITIES GUARANTEE is executed as of the day and year
first above written.

                                       UNOCAL CORPORATION, as Guarantor

                                       By:
                                          ---------------------------
                                          Name:
                                          Title:


                                       THE BANK OF NEW YORK, as Preferred
                                       Guarantee Trustee

                                       By:
                                          ---------------------------
                                          Name:
                                          Title:


                                      -16-