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                                                                    Exhibit 10.4


                       AMENDMENT TO DIAMOND SHAMROCK, INC.
                     SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN


         Ultramar Diamond Shamrock Corporation, a Delaware corporation, pursuant
to authority granted by its Board of Directors, hereby adopts the following
amendments to the Diamond Shamrock, Inc. Supplemental Executive Retirement Plan
(the "DS SERP"). Such amendments shall be effective as of May 1, 2000, subject
to such further limitations and restrictions as are set forth below.

1.  The version of Section 2(a) (defining "Average Monthly Compensation") which
applies to any employee who was designed a Participant in the Plan on or after
December 5, 1995 shall similarly apply to each employee who (i) was designed a
Participant in the Plan prior to December 5, 1995 and (ii) remains employed by
Ultramar Diamond Shamrock Corporation on the date set forth above.

2.  The version of Section 2(b) (defining "Basic Compensation") which applies to
any employee who was designed a Participant in the Plan on or after December 5,
1995 still similarly apply to each employee who (i) was designed a Participant
in the Plan prior to December 5, 1995 and (ii) remains employed by Ultramar
Diamond Shamrock Corporation on the date set forth above.

3.  Section 2(f) is amended and restated, in its entirety, effective for any
person who remains employed by Ultramar Diamond Shamrock Corporation on the date
set forth above, as follows:

         (f) "Change in Control" shall mean the occurrence of any event which
         constitutes a "Change in Control" under the Ultramar Corporation
         Supplemental Executive Retirement Plan.

4.  Section 2(g) is amended and restated, in its entirety, as follows:

         (g) "Corporation" means Ultramar Diamond Shamrock Corporation, a
         Delaware corporation.

5.  Section 4(b) is amended and restated, in its entirety, as follows:

         Notwithstanding the provisions of Section 4(a) hereof, the Corporation
         has established a trust to provide for the payment of benefits due
         under the Plan (hereafter, the "Trust"), under such circumstances
         (including a Change in Control) as are specified under the terms of the
         Trust. The trustee of the Trust is Sterling National Bank and Trust
         Company of New York. To the extent that the Trust does not pay the
         benefits under the Plan to which any Participant (or beneficiary
         thereof) is entitled, the Corporation remains responsible to do so.
         Moreover, all




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         assets of the Trust remain, at all times, subject to the claims of the
         Corporation's creditors in the event of the Corporation's insolvency,
         and no Participant (or any beneficiary thereof) shall, at any time,
         have a prior claim to any Trust assets.

6.  A new final sentence is added to Section 9(d), as follows:

         Notwithstanding any other provision of the Plan to the contrary, the
         foregoing provisions of this subsection (d) shall so apply with respect
         to any Participant (or any beneficiary thereof) only if such
         Participant otherwise consents, in a writing in accordance with the
         requirements of Section 12.11 of such Participant's employment
         agreement with the Corporation, to such application of Dialogue.

7.  A new Section 14 is added, effective for any person who remains employed by
Ultramar Diamond Shamrock Corporation on the date set forth above, as follows:

         14. Special Change in Control Provisions.

                  (a) The provisions of this Section 14 shall apply with respect
         to each Participant in the group listed in subsection (b) hereof (each,
         hereafter, a "Covered Participant") who, except as otherwise provided
         in this Section 14, remains employed by the Corporation (or any
         subsidiary or affiliate of the Corporation) (collectively, "UDS") upon
         the occurrence of a Change in Control (with the date on which such
         Change of Control occurs being hereafter referred to as the "Change in
         Control Date"). The provisions of this Section 14 shall apply,
         notwithstanding any other provision of the Plan to the contrary.

                  (b) The Covered Participants, identified by social security
         number, are as follows:

                                        [William R. Klesse]
                                        [R. S. Beadle]
                                        [Timothy J. Fretthold]
                                        [W. Paul Eisman]
                                        [H. Pete Smith]

                  (c) Each Covered Participant who remains employed with UDS on
         the Change in Control Date shall, within thirty (30) days following
         such date, receive a lump sum distribution on account of such person's
         participation in the Plan, in an amount determined under whichever of
         the following subsections (f), (g) or (h) shall apply with respect to
         such person.

                  (d) The Average Monthly Compensation of each Covered
         Participant who remains employed with UDS on the Change in Control Date
         shall, for all purposes of the Plan, be determined with the following
         modifications:

               (i)  the "considered period" (as that phrase is otherwise used in
                    the definition of Average Monthly Compensation) shall
                    include the calendar year in which the Change in Control
                    Date occurs, as well as each of the two immediately
                    succeeding years,

               (ii) such Covered Participant's Basic Compensation for each of
                    these three calendar years shall be deemed equal to the
                    Basic Compensation paid to such Participant during the
                    calendar year


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                       immediately preceding the calendar year in which the
                       Change in Control Date occurs, and

               (iii)   such Covered Participant's Incentive Compensation earned
                       with respect to each of these three calendar years shall
                       be deemed equal to such Participant's Incentive
                       Compensation earned with respect to the calendar year
                       immediately preceding the calendar year in which the
                       Change in Control Date occurs.

         The aforementioned provisions of this subsection (d) shall also apply
         in determining the amount of any subsequent distribution to which such
         person may otherwise be entitled, except to the extent that such person
         would receive a larger such distribution in the absence of the
         application of this subsection (d) to such subsequent distribution.

                  (e) The following definitions shall apply for purposes of this
         Section 14:

                  (i)  "PBGC Factors" shall, with respect to any Covered
                       Participant, mean (I) the interest rate issued by the
                       Pension Benefit Guaranty Corporation for private sector
                       lump sum payments, as such rate is in effect on the first
                       day of the calendar year in which the Change in Control
                       Date occurs and (II) the Unisex Pension 1984 (1, -3)
                       mortality table.

                  (ii) "GATT Factors" shall, with respect to any Covered
                       Participant, mean the annual interest rate on 30-year
                       United States Treasury securities, as specified by the
                       Commissioner of Internal Revenue, for the month of
                       December in the calendar year preceding the calendar year
                       in which the Change in Control Date occurs and (II) the
                       prevailing commissioners' standard table (described in
                       Section 807(d)(5)(A) of the Code) used to determine
                       reserves for group annuity contracts issued on the Change
                       in Control Date, as set forth in Revenue Ruling 95-6, or
                       any subsequent guidance thereto.

                  (f) For a Covered Participant described in the foregoing
         subsection (c) who, on the Change in Control Date, is at least age 59,
         the lump sum payable to such person under the foregoing subsection (c)
         shall be determined in the following steps, with the lump sum being the
         amount determined under the following clause (ii):

                  (i)  Determine the amount of the monthly Normal Retirement
                       Benefit which would otherwise be immediately payable to
                       such person under Section 6(a), after taking into account
                       the provisions of the foregoing subsection (d), with such
                       benefit calculated as if (A) such person was three years
                       older than such person's then actual age, (B) such person
                       terminated employment on the Change in Control Date and
                       (C) such person's Normal Retirement Date was




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                       also the Change in Control Date; provided, however, that
                       in making such determination, in calculating such Covered
                       Participant's Section 6(a) monthly Normal Retirement
                       Benefit (which is the starting point in calculating such
                       Covered Participant's Early Retirement Benefit), the
                       amount of any particular Other Retirement Benefit taken
                       into account in determining "the sum of his Other
                       Retirement Benefits" (as that phrase is used in Section
                       6(a)) shall be determined with reference to the amount of
                       such particular Other Retirement Benefit which would
                       otherwise be payable to such Covered Participant,
                       starting on the Change in Control Date, had such person
                       both terminated employment and commenced to receive such
                       benefit, as of such date.

                  (ii) Determine the immediate present value, based upon such
                       Covered Participant's then actual age, of the monthly
                       benefit amount, calculated under the foregoing clause
                       (i), which would otherwise be paid to such person,
                       starting on the Change in Control Date, with such present
                       value determined using the PBGC Factors; provided,
                       however, that such present value shall instead be
                       determined using the GATT Factors, to the extent that
                       doing so would result in a larger such immediate present
                       value.

                  (g) For a Covered Participant described in the forgoing
         subsection (c) who, on the Change in Control Date, is less than age 59,
         but no less than three years younger than the age corresponding to such
         person's Early Retirement Date, the lump sum payable to such person
         under the foregoing subsection (c) shall be determined in the following
         steps, with the lump sum being the amount determined under the
         following clause (ii):

                  (i)  Determine the amount of the monthly Early Retirement
                       Benefit which would otherwise be immediately payable to
                       such person under Section 6(b), after taking into account
                       the provisions of the foregoing subsection (d), with such
                       benefit calculated as if (A) such person was three years
                       older than such person's then actual age, (B) such person
                       terminated employment on the Change in Control Date and
                       (C) such person's Early Retirement Date was also the
                       Change in Control Date; provided, however, that in making
                       such determination, in calculating such Covered
                       Participant's Section 6(a) monthly Normal Retirement
                       Benefit:

                           (I) to the extent such Covered Participant is at
                               least age 55 on the Change in Control Date, the
                               amount of such particular Other Retirement
                               Benefit taken into account in determining "the
                               sum of his Other Retirement Benefits" shall be
                               determined with reference to the amount of such
                               particular Other Retirement Benefit which would
                               otherwise be payable to such Covered Participant,
                               starting on the Change in Control Date, had such
                               person both terminated



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                                employment and commenced to receive such
                                benefit, as of such date, and

                           (II) to the extent such Covered Participant is less
                                than age 55 on the Change in Control Date, the
                                amount of such particular Other Retirement
                                Benefit taken into account in determining "the
                                sum of his Other Retirement Benefits" shall be
                                determined with reference to the amount of such
                                particular Other Retirement Benefit which would
                                otherwise be payable to such Covered
                                Participant, starting at age 55, but with the
                                amount otherwise payable at age 55 determined,
                                to the extent otherwise relevant under the plan
                                or program under, or with respect to, which such
                                particular Other Retirement Benefit is otherwise
                                paid, based upon such person's actual age on the
                                Change in Control Date.

                  (ii) Determine the immediate present value, based upon such
                       Covered Participant's then actual age, of the monthly
                       benefit amount, calculated under the foregoing clause
                       (i), which would otherwise be paid to such person,
                       starting on the Change in Control Date, with such present
                       value determined using the PBGC Factors; provided,
                       however, that such present value shall instead be
                       determined using the GATT Factors, to the extent that
                       doing so would result in a larger such immediate present
                       value.

                  (h) For a Covered Participant described in the foregoing
         subsection (c) who, on the Change in Control Date, is more than three
         years younger than the age corresponding to such person's Early
         Retirement Date, the lump sum payable to such person under the
         foregoing subsection (c) shall be determined in the following steps,
         with the lump sum being the amount determined under the following
         clause (iii):

                  (i)  Determine the amount of the monthly Early Retirement
                       Benefit which would otherwise be payable to such person
                       under Section 6(b), after taking into account the
                       provisions of the foregoing subsection (d), commencing
                       with such person's attainment of the age corresponding to
                       such person's Early Retirement Date, calculated as if
                       such person had terminated employment on the Change in
                       Control Date and with the amount of the reduction for
                       early benefit payment under Section 6(b) being determined
                       as of such person were, on the Change in Control Date,
                       exactly the age corresponding to such person's Early
                       Retirement Date; provided, however, that in making such
                       determination, in calculating such Covered Participant's
                       Section 6(a) monthly Normal Retirement Benefit, the
                       amount of any particular Other Retirement Benefit taken
                       into account in determining "the sum of his Other
                       Retirement Benefits" shall be determined with reference
                       to the amount of such particular Other Retirement Benefit
                       which would otherwise be payable to such Covered
                       Participant, starting at age 55, but with the amount
                       otherwise payable at age 55 determined, to the extent




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                        otherwise relevant under the plan or program under, or
                        with respect to, which such particular Other Retirement
                        Benefit is otherwise paid, based upon such person's
                        actual age on the Change in Control Date.

                  (ii)  Determine the actuarial equivalent monthly benefit
                        commencing on the Change in Control Date, and treating
                        such Covered Participant, for this purpose, as if such
                        person was three years older than such person's then
                        actual age, of the monthly benefit amount, calculated
                        under the foregoing clause (i), which would otherwise
                        commence to be paid to such person when such person
                        attains the age corresponding to such person's Early
                        Retirement Date, with such actuarial equivalence
                        determined using the PBGC Factors.

                  (iii) Determine the immediate present value, based upon such
                        Covered Participant's then actual age, of the monthly
                        benefit amount calculated under the foregoing clause
                        (ii), with such present value determined using the PBGC
                        Factors.

         Notwithstanding the foregoing clauses (ii) and (iii), actuarial
         equivalence, for purposes of such clause (ii), and present value, for
         purposes of such clause (iii), shall instead be determined using the
         GATT Factors, but only to the extent that using the GATT Factors under
         both such clauses (ii) and (iii) would result in a larger lump sum
         amount being determined under such clause (iii).

                  (i) Attached to the Plan, as Exhibit A, is a separate schedule
         for each Covered Participant (together with certain supporting
         schedules) illustrating the manner, in accordance with the methodology
         set out in the foregoing provisions of this Section 14, in which the
         lump sum payment described in the foregoing subsection (c) would be
         calculated for such person, assuming that (I) the Change in Control
         Date occurs on December 31, 2000 and (II) such person remains employed
         by UDS on that date. In the event of a Change in Control occurring on
         December 31, 2000, the amount of the lump sum payable under such
         subsection (c) to any Covered Participant who remains employed by UDS
         on such date shall be the amount set forth with respect to such person
         in the relevant attached schedule. In the event that a Change in
         Control Date occurs on some other date, the methodology set out in such
         schedules (including such supporting schedules) shall be dispositive in
         resolving any issues which may arise in connection with determining the
         amount of the lump sum provided under the foregoing subsection (c) and
         otherwise payable to any such Covered Participant with respect to such
         other Change in Control Date. Notwithstanding the foregoing, in the
         case of that Covered Participant identified under subsection (l) of
         this Section 14, the amount set forth in such person's schedule shall
         be reduced in accordance with the provisions of such subsection (l).

                  (j) If a Covered Participant who receives a lump sum
         distribution pursuant to the provisions of the foregoing subsection
         (f), (g) or (h), as the case may be, continues to be employed by UDS
         and thereafter becomes entitled to a subsequent distribution with
         respect to the Plan, such person's





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         monthly Normal Retirement Benefit, for purposes of determining the
         amount, if any, of such subsequent distribution, shall be equal to the
         excess of:

                  (i)   the amount of such monthly Normal Retirement Benefit,
                        determined by disregarding the prior lump sum
                        distribution pursuant to the foregoing subsections (f),
                        (g) or (h), as the case may be, over,

                  (ii)  the amount of the monthly Normal Retirement Benefit
                        which was taken into account under the foregoing
                        subsection (f), (g) or (h), as the starting point in
                        determining the amount of such prior lump sum determined
                        under such respective subsection.

         Additionally, for purposes of applying the provisions of Section 6(b)
         with respect to such subsequent distribution, such person shall be
         treated as being three years older than such person's actual age. In
         all other respects, the amount of any subsequent distribution shall be
         determined in accordance with such rules of uniform application as may
         be established by the Employee Benefits Committee.

                  (k) Notwithstanding any of the foregoing provisions of this
         Section 14 to the contrary, in the event of a Covered Participant's
         "involuntary termination, other than for Cause" (as those terms are
         defined under such person's employment agreement with the Corporation),
         in anticipation of a Change in Control:

                  (i)   the foregoing provisions of this Section 14 shall apply
                        to such person,

                  (ii)  pursuant thereto, a Change in Control so triggering the
                        application of the foregoing provisions of this Section
                        14 with respect to such person shall, solely with
                        respect to such person, be considered to have occurred
                        on the date immediately preceding the date on which such
                        person is so terminated from employment, and

                  (iii) such person's benefit under the UDS Pension Plan, for
                        purposes of determining "the sum of his Other Retirement
                        Benefits", shall be computed by including the additional
                        years of age and service credit which were (or will be)
                        taken into account, pursuant to the provisions of
                        Section 5.5(i)(a)(3) of such person's employment
                        agreement with the Corporation, in computing the amount
                        of the lump sum payment made (or to be made) to such
                        person in lieu of an actual increase in such person's
                        benefit under the UDS Pension Plan.

                  (l) Notwithstanding the foregoing provisions of this Section
         14, any lump sum distribution otherwise payable to that Covered
         Participant whose social security number is [H. Pete Smith], as
         otherwise determined under the foregoing provisions of this Section 14,
         shall be reduced by the amount of any lump sum distribution otherwise
         paid (or payable) to such person under the Ultramar Corporation
         Supplemental Executive Retirement Plan.




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                  (m) Each Covered Participant who remains employed with UDS on
         the Change in Control Date, determined without regard to the foregoing
         subsection (k), shall receive a lump sum payment in the amount
         specified below, upon the earlier of:

                  (i)  such person's "involuntary termination, other than for
                       Cause," as those terms are defined under the employment
                       agreement between such person and the Corporation;
                       provided, however, that an "involuntary termination"
                       shall not be deemed to have occurred for purposes of this
                       clause (i) in the event that such person voluntarily
                       terminates employment on account of a significant
                       reduction, occurring not later than the Change in Control
                       Date, in such person's duties or the addition, occurring
                       not later than the Change in Control Date, of duties
                       which, in either case, are materially inconsistent with
                       such person's then title or position, such that no amount
                       shall be paid pursuant to this subsection (m) to such
                       person; and further, provided, however, that an
                       "involuntary termination" shall be deemed to have
                       occurred for purposes of this clause (i) in the event
                       that such person voluntarily terminates employment on
                       account of a significant reduction, occurring subsequent
                       to the Change in Control Date, in such person's duties or
                       the addition, occurring subsequent to the Change in
                       Control Date, of duties which, in either case, are
                       materially inconsistent with such person's title or
                       position as in effect on the Change in Control Date, such
                       that an amount shall be paid pursuant to this subsection
                       (m) to such person, or

                  (ii) twelve months following the Change in Control Date,
                       provided such person is still employed by UDS on such
                       date.

         No amount shall be payable pursuant to this subsection (m) in the case
         of any Covered Participant who terminates employment prior to the date
         set forth in the foregoing clause (ii) for any reason not described in
         the foregoing clause (i).

                  Any amount otherwise payable pursuant to this subsection (m)
         to any Covered Participant (i) shall not be reduced by any amounts
         previously paid to such person pursuant to any other provision of the
         Plan and (ii) shall be disregarded in determining the amount of any
         future benefits otherwise payable to such person pursuant to any other
         provision of the Plan.

                  The amount payable to each such Covered Participant, assuming
         such person otherwise meets the requirements of the foregoing
         provisions of this subsection (m) is the amount set forth opposite such
         person's social security number, as follows:

                                    [William R. Klesse        $500,000]
                                    [R. S. Beadle             $250,000]
                                    [Timothy J. Fretthold     $500,000]
                                    [W. Paul Eisman           $250,000]

                                    Ultramar Diamond Shamrock Corporation


                                    By: _______________________________