Filed by Ultramar Diamond Shamrock Corporation
              Pursuant to Rule 425 under the Securities Act of 1933
                    and deemed filed pursuant to Rule 14a-12
                    under the Securities Exchange Act of 1934

             Subject Company: Ultramar Diamond Shamrock Corporation
                           Commission File No. 1-11154




On May 7, 2001,  Valero  Energy  Corporation  ("Valero")  and  Ultramar  Diamond
Shamrock   ("UDS")   issued  the  following   press  release  and   supplemental
information.

                                     * * * *

The enclosed materials contain forward-looking  statements within the meaning of
the "safe harbor" provisions of the Private Securities  Litigation Reform Act of
1995.  These  statements  are based on  management's  current  expectations  and
beliefs  and are  subject to a number of factors  and  uncertainties  that could
cause  actual  results  to  differ   materially  from  those  described  in  the
forward-looking  statements.  The forward-looking  statements  contained in this
release include  statements about future financial and operating results and the
proposed Valero/UDS merger.

The  following  factors,  among  others,  could cause  actual  results to differ
materially from those described in the forward-looking statements: the risk that
Valero's and UDS's businesses will not be integrated successfully; costs related
to the merger;  failure of the Valero or UDS stockholders to approve the merger;
and other economic,  business,  competitive  and/or regulatory factors affecting
Valero's  and UDS's  businesses  generally  as set forth in  Valero's  and UDS's
filings with the  Securities  and Exchange  Commission,  including  their Annual
Reports  on  Form  10-K  for the  fiscal  year  ended  2000,  especially  in the
Management's  Discussion  and  Analysis  section,  their most  recent  Quarterly
Reports on Form 10-Q and their Current  Reports on Form 8-K.  Valero and UDS are
under no obligation to (and expressly disclaim any such obligation to) update or
alter their  forward-looking  statements whether as a result of new information,
future events or otherwise.

INVESTORS   AND   SECURITY   HOLDERS   ARE  URGED  TO  READ  THE   JOINT   PROXY
STATEMENT/PROSPECTUS  REGARDING THE BUSINESS COMBINATION  TRANSACTION REFERENCED
IN THE FOREGOING INFORMATION, WHEN IT BECOMES AVAILABLE, BECAUSE IT WILL CONTAIN
IMPORTANT INFORMATION.  The joint proxy  statement/prospectus will be filed with
the Securities and Exchange Commission by Valero and UDS. Investors and security
holders may obtain a free copy of the joint proxy  statement/prospectus (when it
is available) and other documents filed by Valero and UDS with the Commission at
the Commission's web site at www.sec.gov.  The joint proxy  statement/prospectus
and these other documents may also be obtained for free from Valero and UDS.

READ THE DEFINITIVE JOINT PROXY  STATEMENT/PROSPECTUS  CAREFULLY BEFORE MAKING A
DECISION CONCERNING THE MERGER.

Valero and UDS, and their respective  directors,  executive officers and certain
other of their  respective  employees,  may be  soliciting  proxies  from  their
respective  stockholders  in favor of the  approval of the  merger.  Information
regarding the persons who may, under SEC rules,  be deemed to be participants in
the solicitation of Valero and UDS stockholders in connection with the merger is
set forth,  in the case of Valero,  in  Valero's  proxy  statement  for its 2001
annual meeting, filed with the SEC on March 28, 2001, and in the case of UDS, in
UDS's proxy statement for its 2001 annual  meeting,  filed with the SEC on March
27, 2001, and additional  information  will be set forth in the definitive proxy
statement/prospectus referred to above when it is filed with the SEC.

                                     * * * *

THE  FOLLOWING  IS A PRESS  RELEASE  ISSUED BY  VALERO  ENERGY  CORPORATION  AND
ULTRAMAR DIAMOND SHAMROCK ON MAY 7, 2001:

FOR IMMEDIATE RELEASE      CONTACTS:
- --------------------------------------      -----------------
                       VALERO ENERGY CORPORATION           UDS
                       Mary Rose Brown (Media)             Tara Ford (Media)
                       (210) 370-2314                      (210) 592-4163
                       Lee Bailey (Investor Relations)     Scott Spendlove
                       (210) 370-2139                      (Inv. Rel.)
                                                          (210) 592-4019

         VALERO ENERGY CORP. TO ACQUIRE ULTRAMAR DIAMOND SHAMROCK CORP.

      $6 BILLION TRANSACTION TO CREATE SECOND-LARGEST U.S. REFINING COMPANY

SAN ANTONIO,  May 7, 2001 - Valero Energy Corp. (NYSE: VLO) and Ultramar Diamond
Shamrock Corp.  (NYSE:  UDS) jointly announce that the companies have reached an
agreement for Valero to acquire UDS,  which will make Valero the  second-largest
United States  refiner of petroleum  products.  The new  organization  will have
23,000  employees in the United  States and Canada,  13  refineries  and a total
throughput  capacity of just under 2 million barrels per day (BPD).  Valero will
also be one of the  nation's  largest  retailers  with  more than  5,000  retail
outlets in the U.S. and Canada.

With this acquisition, Valero will have annual revenues of $32 billion and total
assets of more than $10 billion. In terms of refining capacity,  Valero will now
be second only to ExxonMobil.

The  transaction is expected to be highly  accretive to earnings,  cash flow and
returns based on current First Call consensus estimates.

"We're  combining the two best independent  refining and marketing  companies to
make the premier refiner and marketer in the U.S.," said Bill Greehey,  Valero's
chairman of the board and CEO. "Obviously this will bring tremendous benefits to
both of our  organizations  and to our  shareholders.  In fact,  in refining and
marketing,  we will be the only major  independent  of a size and scope equal to
the majors.

"Combining  Valero's  complex  refining  system and the  extensive UDS refining,
logistics and retail network gives us a superior asset portfolio that will allow
us to  effectively  compete  in this  rapidly  consolidating  business.  We will
realize  tremendous  synergies and strategic  benefits while enhancing  earnings
stability.  And, we are fortunate to bring aboard such a high-quality management
team and workforce that is among the best in the industry."

The total value of the transaction is approximately  $6 billion,  which includes
approximately  $4 billion in equity  and $2 billion in assumed  debt.  The total
consideration  to be paid to UDS  stockholders in the  transaction  equates to a
fixed  exchange  ratio of 1.228 shares of Valero common stock for  approximately
half of the outstanding  shares of UDS common stock, and $55 dollars in cash per
share for the remaining shares of UDS common stock. The cash  consideration  and
the stock  consideration per UDS share each represent an approximate 30% premium
to UDS  shareholders  based on the 10-day  average per share prices ending April
26th.  UDS  shareholders  will be able to elect to receive either cash or Valero
common stock for each share of UDS common stock they own, subject to pro-ration,
with the consideration,  whether paid in cash or stock,  having a value equal to
the average of $55.00 and the value of 1.228 shares of Valero  common stock over
a measurement period prior to closing.

"This really is a winning  combination for our stockholders,"  said Jean Gaulin,
chairman  of the  board  and  CEO of  UDS.  "UDS  shareholders  will  receive  a
substantial premium,  which reflects a more appropriate  valuation for the stock
and captures  value that the market has otherwise  been slow to recognize.  They
will also lock in a solid return with the cash portion. Valero shareholders will
benefit from a transaction  that is immediately and  significantly  accretive to
earnings and cash flow per share.  Since UDS shareholders will own 40 percent of
the  company,  both  sets  of  shareholders  will  benefit  from  the  increased
profitability  of the company going forward.  And because larger  companies have
more liquidity and  visibility,  we are optimistic that the market will assign a
higher multiple valuation to the new company."

REFINING AND LOGISTICS ASSETS

With  the  addition  of the UDS  refining  assets,  Valero  will  have  the most
geographic diversity among U.S. refiners. The seven UDS refineries, which have a
combined  capacity of 850,000 BPD,  will expand  Valero's  presence in Texas and
California,  while giving it new  locations  in  Colorado,  Oklahoma and Quebec,
Canada.  Like Valero,  the UDS  refineries  are also  generally  high-conversion
facilities which produce premium,  cleaner-burning fuels including  reformulated
gasoline,  California's  cleaner-burning  CARB gasoline and CARB diesel,  all of
which sell at a premium  to  conventional  gasoline.  Valero  will also  acquire
Ultramar Diamond Shamrock's extensive,  4,600-mile proprietary pipeline network,
including the Shamrock  Logistics  MLP and  associated  assets,  that reduce the
refineries' feedstock supply costs and distribute products at a higher netback.

"We will benefit from enhanced  financial  performance  through  realization  of
numerous  potential  synergies  among the facilities,  including  multi-refinery
purchasing and inventory  optimization,"  said Greehey.  "We estimate that these
synergies will have a benefit in excess of $200 million annually."



GREATER EXPOSURE TO POSITIVE FUNDAMENTALS

The transaction also comes amid the very strong refining  industry  fundamentals
that have helped both companies achieve record earnings recently.

"The current industry  fundamentals  and the long-term  outlook for our business
have never been better," said Greehey.  "Refined product inventories continue to
trend at  historically  low levels.  At the same time,  gasoline and  distillate
demand remain strong.  With limited excess refining  capacity in the U.S., there
is very little room for inventories to build  substantially  in the near future.
This should continue to keep the supply/demand balance tight and support healthy
refining margins going forward.

"We continue to see gasoline demand better than last year's levels and expect to
see a good driving season," he added.  "And now more than ever, Valero is poised
to seize the greatest benefits from these fundamentals  because we will continue
to have the most leverage to improved refining margins."

MARKETING ASSETS

Branded Ultramar,  Diamond  Shamrock,  Beacon and Total, UDS has more than 2,500
company-owned  sites  throughout the West,  Southwest and  Mid-continent  of the
United States and in Eastern Canada, which are among the fastest growing markets
in the country. The company also supplies approximately 2,500 dealer, truck stop
and cardlock sites primarily in the Southwest and  Mid-continent  regions of the
United States and in Eastern Canada.

The  company-owned   stores  have  extensive  brand  support  programs  such  as
proprietary consumer and fleet credit cards, radio and television brand support,
and strong in-store marketing programs,  to which Valero will be able to add its
350-store retail network in California.

In addition,  UDS operates  one of the largest  home heating oil  businesses  in
North America that sells heating oil to approximately 250,000 households.

"We're excited to substantially  grow our retail presence because retail margins
are  counter-cyclical  to refining margins,  so in the event we experience lower
refining  margins,  retail  margins  will help  stabilize  our  earnings,"  said
Greehey.  "We are  fortunate  because  UDS has  created  one of the best  retail
networks in the country with excellent leadership and strategic vision. And with
the size and quality of their retail network,  we will benefit from the earnings
stability that can be achieved by having retail  exposure  without  diluting the
leverage  we have to  refining  margins.  So we will truly have the best of both
worlds."

TIMING, TRANSITION AND FUTURE

Greehey  will  remain  chairman  and CEO of Valero  and the  company's  board of
directors  will grow to 13  members  with the  addition  of four UDS  directors.
Gaulin will  continue to run UDS until the closing and will work with Greehey on
the  organization of the new Valero.  He will then retire and step down from the
board of directors.

The boards of directors of both companies have approved the  transaction,  which
is  subject to the  approval  of both  companies'  shareholders  and  regulatory
approvals.  The  acquisition  is  expected  to  close by the end of the year and
Greehey noted that a smooth  transition  is  anticipated.  Morgan  Stanley & Co.
Incorporated  is serving as Valero's  financial  advisor and provided a Fairness
Opinion  letter  related to the  transaction,  and Credit Suisse First Boston is
also providing a Fairness Opinion letter to Valero.  Banc of America Securities,
LLC is advising UDS.

"Valero has  demonstrated  that we deliver on the  transactions  we announce and
that we integrate those  acquisitions  smoothly and effectively,"  Greehey said.
"The fact that UDS is in San Antonio will  contribute to making this  transition
even  easier.  The  combined  company  will benefit from having the best of both
management teams and the assets to build upon for the future.

"And, we're not done! With the financial capacity we'll have, we fully intend to
expand our  business,  grow the  company and  continue  to deliver  value to our
shareholders."

VALERO ENERGY CORPORATION

Valero is a Fortune 500 company based in San Antonio,  with approximately  3,100
employees and 2000 revenues of nearly $15 billion.  The company  currently  owns
and operates six refineries in Texas, Louisiana,  New Jersey and California with
a combined  throughput capacity of more than 1 million BPD. Valero is recognized
throughout   the   industry   as  a  leader  in  the   production   of  premium,
environmentally  clean  products such as  reformulated  gasoline,  CARB Phase II
gasoline,  low-sulfur diesel and oxygenates. The company markets its products in
34 states through an extensive bulk and rack  marketing  network,  in California
through  approximately 350 retail  locations,  and in selected export markets in
Latin America.

ULTRAMAR DIAMOND SHAMROCK CORPORATION

Ultramar Diamond Shamrock Corp. has approximately $17 billion in annual revenues
and more than 20,000  employees.  The company  operates seven  refineries in the
United States and Canada with a total throughput capacity of 850,000 barrels per
day and has nearly 5,000 branded retail gasoline/convenience merchandise stores,
the majority of which are branded Diamond Shamrock,  Ultramar,  Beacon or Total.
The corporation also has growing petrochemicals and home heating oil businesses.

WEBCAST - An analyst meeting is scheduled for 10 a.m. EDT Monday, May 7, at the
- -------
St. Regis Hotel in New York City. To listen, go to www.valero.com or
 --------------------
www.udscorp.com, enter the Investor Relations page and click on the "Live
- -----------------------
Webcast"  link. A replay of this  broadcast  will be available on both  websites
following the presentation.

SATELLITE FEED - a satellite feed will be available on Monday, May 7 at 12-12:15
- --------------
p.m.  (EDT) and 5-5:15 p.m.  (EDT) on Galaxy 4R/C20 and K16. The video  includes
sound  bites  from the CEO's of both  companies,  as well as b-roll of  Valero's
refineries  and  Ultramar  Diamond   Shamrock's  retail  stores.  For  satellite
assistance, call (210) 377-8666.
THE FOLLOWING IS A TRANSACTION SUMMARY DISSEMINATED BY VALERO ENERGY CORPORATION
AND ULTRAMAR DIAMOND SHAMROCK ON MAY 7, 2001 WITH THE FOREGOING PRESS RELEASE:

 [VALERO ENERGY CORPORATION LOGO]               [ULTRAMAR DIAMOND SHAMROCK LOGO]


Bill Greehey                                      Jean Gaulin
Chairman and CEO                                  Chairman and CEO



                             THE RISE OF A REFINING
                                & MARKETING MAJOR

                                    MAY 2001



- ---------------------------------------------------
                                  BILL GREEHEY
                                CHAIRMAN AND CEO
                            VALERO ENERGY CORPORATION


TRANSACTION SUMMARY
- ---------------------------------------------------


  -  VALERO TO ACQUIRE UDS IN OVER $6 BILLION TRANSACTION

  -  EXCHANGE  RATIO FIXED AT 1.228 SHARES OF VALERO STOCK FOR EACH SHARE OF UDS
     STOCK - REPRESENTS  30% PREMIUM  BASED ON TRAILING 10 DAYS  AVERAGE  ENDING
     APRIL 26

  -  TO BE PAID -

      +  50% IN VALERO STOCK

      +  50% IN CASH FIXED AT $55 PER SHARE OF UDS STOCK

  -  TRANSACTION SIGNIFICANTLY ACCRETIVE TO VALERO'S EPS AND
     CFPS

- -        CLOSING BY YEAR END 2001

STRATEGIC RATIONALE
- ---------------------------------------------------


  -  SIZE AND SCOPE CREATES THE PREMIER U.S. REFINER &
     MARKETER

  -  SUPERIOR ASSET PORTFOLIO

      +  QUALITY UDS MANAGEMENT TEAM AND WORKFORCE

  -  ENHANCED EARNINGS STABILITY THROUGH TOP-QUALITY,
     NATIONALLY RECOGNIZED RETAIL OPERATIONS

  -  ESTIMATED SYNERGIES OF AT LEAST $200 MILLION ANNUALLY

  -  SIGNIFICANTLY INCREASED SIZE AND LIQUIDITY -- MULTIPLE
     --------------
     EXPANSION
     -----------------

      +  LIKELY INCLUSION IN S&P 500

- -        POSITIONS COMPANY FOR FURTHER GROWTH

THE COMBINED COMPANY
- ---------------------------------------------------


  -  BILL GREEHEY TO REMAIN CHAIRMAN AND CEO OF
     VALERO

  -  VALERO BOARD TO EXPAND TO 13 MEMBERS WITH
     ADDITION OF 4 UDS DIRECTORS

           -------------------------------------------------------------
           |                                                             |
           |              HEADQUARTERS IN SAN ANTONIO
           |                                                             |
           |                    23,000 EMPLOYEES
           |                                                             |
           |                     13 REFINERIES
           |                                                             |
           |                  5,350 RETAIL OUTLETS
           |                                                             |
           |        4,600 MILES OF CRUDE & PRODUCT PIPELINES
           |                                                             |
           |                $32 BILLION IN REVENUES
           |                                                             |
           |              $10 BILLION IN TOTAL ASSETS
           |                                                             |
           -------------------------------------------------------------

THE NEW U.S. REFINING MAJOR
- ---------------------------------------------------


[BAR GRAPH DEPICTING THE FOLLOWING DATA:

                               Capacity
Company                        (bpd)
- ---------------                ---------
ExxonMobil                          1,955
Valero/UDS                          1,865
Phillips/Tosco                      1,710
BP                                  1,670
Chevron/Texaco                      1,520
Valero                              1,015
MAP                                   935
UDS                                   850
Shell                                 825
Sunoco                                730
El Paso                               540
Conoco                                530
Premcor                               480
Tesoro                                275


Source:  Oil & Gas Journal and company reports

WITH MAJOR MARKETING PRESENCE
- ---------------------------------------------------


                                                           Company
                                                           Operated
    Rank/Company                                           U.S. Sites
    ---------------------------------------------------------------
    1.  Phillips Petroleum                                 2,350
    2.  Speedway SuperAmerica (MAP)            2,200
    3.  The Southland Corp.                                2,050
 -------------------------------------------------------------------
 |  4.  VALERO ENERGY CORPORATION   1,735    |
 -------------------------------------------------------------------
    5.  BP/Amoco/Arco                                      1,635
    6.  The Pantry Inc.                                    1,200
    7.  ExxonMobil                                         1,075
    8.  Equilon/Motiva                                        975
    9.  Clark                                                 900
    10. Cumberland Farms, Inc.                               725



   Source:  2000 C-Store & Pet. Mktg. Industry Review, 1999 data adjusted
            by UDS management


- --------------------------------------------------
                                   JEAN GAULIN
                                CHAIRMAN AND CEO
                            ULTRAMAR DIAMOND SHAMROCK

GEOGRAPHIC DIVERSITY AND COMPLEXITY ...
- ---------------------------------------------------


[MAP OF THE  UNITED  STATES OF AMERICA  AND  SOUTHEASTERN  CANADA  DEMONSTRATING
VALERO'S RETAIL MARKETING  PRESENCE IN THE STATE OF CALIFORNIA AND THE FOLLOWING
INFORMATION RELATED TO VALERO'S REFINERIES:

Benicia, California
  -  165,000 bpd capacity
  -  14.5 complexity

Corpus Christi, Texas
  -  225,000 bpd capacity
  -  23.8 complexity

Texas City, Texas
  -  230,000 bpd capacity
  -  9.2 complexity

Houston, Texas
  -  120,000 bpd capacity
  -  9.9 complexity

Krotz Springs, Louisiana
  -  85,000 bpd capacity
  -  5.8 complexity

Paulsboro, New Jersey
  -  185,000 bpd capacity
  -  12.1 complexity

 .. ENHANCED BY UDS' QUALITY ASSETS
- ---------------------------------------------------


[MAP OF THE UNITED STATES OF AMERICA AND SOUTHEASTERN CANADA DEMONSTRATING
VALERO AND UDS COMBINED RETAIL MARKETING PRESENCE, INFORMATION RELATED TO VALERO
AND UDS  REFINERIES,  AND  LOCATION  OF  TERMINALS,  CRUDE  PIPELINES,  PRODUCTS
PIPELINES, INDUSTRY PIPELINES AND ETHYLENE/PROPYLENE PIPELINES:

COMBINED MARKETING PRESENCE IN THE FOLLOWING STATES AND PROVINCES:

United States:               Canada (Southeastern region only):
- --------------                  ----------------------------------
Arizona                         New Brunswick
Arkansas                       Ontario
California                      Quebec
Colorado
Iowa
Kansas
Louisiana
Maine
Massachusetts
Minnesota
Missouri
Nebraska
New Hampshire
New Mexico
Oklahoma
Texas
Vermont
Wyoming


REFINERIES:

Valero:                                         UDS:
- -------                                          ------
Benicia, California                     Golden Eagle, California
  -  165,000 bpd capacity                      -  168,000 bpd capacity
  -  14.5 complexity                           -  11.5 complexity

Corpus Christi, Texas                   Wilmington, California
  -  225,000 bpd capacity                      -  135,000 bpd capacity
  -  23.8 complexity                           -  16.8 complexity

Texas City, Texas                       McKee, Texas
  -  230,000 bpd capacity                      -  170,000 bpd capacity
  -  9.2 complexity                            -  8.0 complexity

Houston, Texas                          Three Rivers, Texas
  -  120,000 bpd capacity                      -  98,000 bpd capacity
  -  9.9 complexity                            -  10.2 complexity

Krotz Springs, Louisiana                Ardmore, Oklahoma
  -  85,000 bpd capacity                       -  85,000 bpd capacity
  -  5.8 complexity                            -  9.5 complexity

Paulsboro, New Jersey                   Denver, Colorado
  -  185,000 bpd capacity                      -  27,000 bpd capacity
  -  12.1 complexity                           -  6.0 complexity

                                                       Quebec, Canada
                                               -  167,000 bpd capacity
                                               -  7.2 complexity

BETTER EXPOSURE TO THE RIGHT MARKETS
- ---------------------------------------------------

[BAR GRAPH DEPICTING THE FOLLOWING DATA:

                Composite Average Refining Spreads*
                                     ($/BBL)
                ------------------------------------------------
Year            Valero Before    Valero After
- -------            -------------          ------------
1993                    4.30                  6.10
1994                    3.84                  5.10
1995                    3.58                  4.69
1996                    4.06                  5.41
1997                    4.43                  5.78
1998                    3.62                  4.79
1999                    3.27                  4.79
2000                    6.27                  8.28
2001                    7.72                  9.81

* 5-3-2  refining  margins  (5  parts  crude  oil,  3  parts  gasoline,  2 parts
  distillate)


ENHANCING THE STABILITY OF EARNINGS
- ---------------------------------------------------

[LINE GRAPH DEPICTING THE FOLLOWING DATA:

3-Month Rolling Average

                                Gulf Coast                  UDS Mid-Continent
                            Refining Spreads            Retail Fuel Margins
Year                          ($/BBL)                       (CENTS/GAL)
- ----                         ----------------           -----------------------
Mar 1997                            3.4                                13.3
Apr 1997                            3.8                                13.4
May 1997                            3.7                                13.2
Jun 1997                            3.6                                13.7
Jul 1997                            3.5                                13.9
Aug 1997                            4.1                                12.9
Sep 1997                            4.2                                12.3
Oct 1997                            3.6                                11.9
Nov 1997                            2.6                                11.7
Dec 1997                            2.3                                10.8
Jan 1998                            2.5                                 12
Feb 1998                            2.6                                12.8
Mar 1998                            2.6                                14
Apr 1998                            2.9                                13
May 1998                            3.4                                12.4
Jun 1998                            3.9                                12.6
Jul 1998                            3.6                                13.3
Aug 1998                            2.9                                14.9
Sep 1998                            2.3                                15.8
Oct 1998                            2.3                                15
Nov 1998                            2.2                                15.2
Dec 1998                            1.9                                15.6
Jan 1999                            1.3                                15.5
Feb 1999                            1.1                                13.5
Mar 1999                            1.4                                10.8
Apr 1999                            1.8                                10
May 1999                            2                                  10.7
Jun 1999                            1.8                                12.1
Jul 1999                            1.8                                11.8
Aug 1999                            2.5                                10.9
Sep 1999                            2.8                                10
Oct 1999                            2.6                                11.4
Nov 1999                            2.3                                11.7
Dec 1999                            2                                  10.9
Jan 2000                            2.2                                 9.6
Feb 2000                            2.8                                 7.8
Mar 2000                            3.7                                 7.5
Apr 2000                            4.5                                 7.9
May 2000                            5.1                                 8.9
Jun 2000                            5.5                                 8.9
Jul 2000                            5.3                                10
Aug 2000                            4.9                                10.7
Sep 2000                            4.8                                10.7
Oct 2000                            5                                   8.8
Nov 2000                            4.6                                 7.4
Dec 2000                            4.1                                 8.4
Jan 2001                            4.6                                 8.4
Feb 2001                            5                                   8.6
Mar 2001                            5.2                                 7.3
Apr 2001                            6.1                                 6.7


- --->  Retail fuel margins tend to be counter-cyclical to refining spreads

QUALITY RETAIL PRESENCE
- ---------------------------------------------------

[UDS SERVICES STATION GRAPHIC]

- -  SOME 2,000 COMPANY-OPERATED SITES AND OVER
   2,500 DEALERS/JOBBERS AND TRUCK RACK FACILITIES
   IN 18 U.S. STATES AND SIX CANADIAN PROVINCES

        -  LARGEST COMPANY-OWNED RETAILER IN TEXAS,
           OKLAHOMA, COLORADO AND NEW MEXICO

        -  ONE OF THE LARGEST HOME HEATING OIL
           BUSINESSES IN NORTH AMERICA

        -  OVER $1.2 BILLION IN ANNUAL MERCHANDISE
           SALES AND GROWING

[VALERO SERVICES STATION GRAPHIC]

- -  80 VALERO BRANDED SITES AND 260 EXXON-
   BRANDED LOCATIONS IN THE CALIFORNIA MARKET

- -        30% INCREASE IN VOLUMES SINCE ACQUISITION

IMPROVING OPERATIONS ...
- ---------------------------------------------------
 ... THROUGH HIGH RETURN PROJECTS ADDING OVER $175 MILLION ANNUALLY


- -  RESTART OF NO. 3 CRUDE UNIT AT THE GOLDEN EAGLE REFINERY

- -  INCREASING CARB GASOLINE AND DIESEL PRODUCTION AT BOTH
   GOLDEN EAGLE AND WILMINGTON TO NEARLY 90%

- -  EXPANSION OF QUEBEC REFINERY BY 40,000 BPD LATER THIS YEAR

- -  ENHANCING SOUR CRUDE AND CLEAN PRODUCT CAPABILITIES AT
   MCKEE, THREE RIVERS AND ARDMORE

- -  COST REDUCTION AND RE-IMAGING PROGRAMS IN PROGRESS
   THROUGHOUT THE RETAIL NETWORK

- -  EXPANDING CRUDE OIL AND PRODUCT LOGISTICS THROUGHOUT MID-
   CONTINENT
- --------------------------------------------------
                                  BILL GREEHEY
                                CHAIRMAN AND CEO
                            VALERO ENERGY CORPORATION

BETTER REFINERIES, BETTER PRODUCTS
- ---------------------------------------------------

- -  2000 UPGRADES AT TEXAS CITY, BENICIA AND PAULSBORO
   TO CONTRIBUTE $40 TO $50 MILLION ANNUALLY

- -  1ST QTR. 2001 PROJECTS - $50 MILLION ANNUAL BENEFIT

        -  BENICIA HYDROCRACKER - INCREASED CARB PRODUCTION

        -  HOUSTON FCC AND PLANTWIDE TURNAROUND - UP TO 100%
           SOUR CRUDE AND LIGHT PRODUCT YIELDS

        -  TEXAS CITY CRUDE UNIT UPGRADE - INCREASED SOUR CRUDE
           CAPACITY AND LIGHT PRODUCT YIELDS


                ---->  MORE HIGH RETURN
                       PROJECTS TO COME

ANTICIPATED SYNERGIES
- ---------------------------------------------------
($ IN MILLIONS)



                                            YEAR 1        YEAR 2+
                                           --------      ---------
CRUDE SOURCING & LOGISTICS                   $ 75          $ 80

MARKETING                                      60            75

ADMINISTRATIVE, PROCUREMENT &
BEST PRACTICES                                 55            85
                                           ----------     -----------
                                 TOTAL       $190          $240


- ----> SIGNIFICANT UPSIDE TO CURRENT ESTIMATES

RIGHT DEAL AT THE RIGHT TIME
- ---------------------------------------------------

- -  INDUSTRY FUNDAMENTALS REMAIN POSITIVE

- -  INVENTORIES FOR LIGHT PRODUCTS SHOULD REMAIN AT LOW END OF
   HISTORIC RANGE

        -  HIGH NATURAL GAS PRICES IMPACT INVENTORIES OF BOTH GASOLINE
           AND DISTILLATE

        -  CONTINUED HEALTHY DEMAND FOR REFINED PRODUCTS

        -  TIGHT US REFINING CAPACITY; LIMITED CAPACITY GROWTH

        -  WORLDWIDE MOVEMENT TOWARD CLEANER BURNING FUELS
           ACCELERATING

- -  POSITIVE SOUR CRUDE FUNDAMENTALS


        ---->  PERFECT CYCLE TO CONTINUE

COMPELLING FINANCIAL IMPACT
- ---------------------------------------------------


                                            2001 Forward Curve Case (1)
                                    ------------------------------------------
                                     Valero            UDS         Pro Forma
                                    ------------------------------------------
    EBITDA                           $1,301         $1,413          $2,714

    Net Income                         $626           $579          $1,263

    Earnings Per Share                $9.73          $7.86          $11.46

      Accretion ($)                      --             --           $1.73

    Cash Flow Per Share              $15.43         $14.84          $17.19

      Accretion ($)                      --             --           $1.76

    Long Term Debt                   $1,041         $1,306          $4,104

    Debt-to-capitalization            31.8%          44.0%           45.6%

    Fixed charge coverage             11.1x           8.6x            6.2x

* Pro Forma includes acquisition financing, synergies and purchase accounting
  adjustments

(1)      Based on 2001 forward curve prices as of April 16, 2001

BASE EARNINGS RAISED SUBSTANTIALLY
- ---------------------------------------------------

ESTIMATED EPS CASES
- ---------------------------------

                                           2002         5 year Avg.
                            2001         First Call        Case
                          ---------      ----------     -----------
Valero                     $9.73           $4.70           $4.84

Pro Forma                 $11.46           $7.08           $7.85

Accretion                    18%             51%             62%


    +  ACCRETION INCREASES AS REFINING MARGINS DECLINE

    +  DRAMATIC REDUCTION IN VOLATILITY

ESTIMATED TRANSACTION TIMETABLE
- ---------------------------------------------------


MAY 7-16                ROADSHOW

MAY 31                  UDS & VLO TO FILE HSR WITH FTC

JUNE 4-7                FILE REGISTRATION AND PROXY STATEMENTS
                                    WITH SEC

SEPTEMBER 6             STOCKHOLDER MEETINGS OF VALERO AND UDS
                        TO APPROVE TRANSACTION

OCTOBER 31              CLOSING

A NEW "MAJOR" POSITIONED FOR GROWTH
- ---------------------------------------------------


- -  THE ONLY "MAJOR" U.S. INDEPENDENT REFINER AND MARKETER

        -  SECOND LARGEST U.S. REFINER, INCLUDING THE MAJORS

        -  CLEAR SEPARATION FROM INDEPENDENT REFINERS

- -  POSITIONED TO CAPITALIZE ON THE FUTURE OF THE R&M INDUSTRY

- -  MAINTAINING HIGHEST LEVERAGE TO QUALITY REFINING MARGINS
   WITH GEOGRAPHICALLY DIVERSE AND RELIABLE OPERATIONS

- -  HIGH LEVERAGE TO STABILIZING RETAIL MARGINS AND GROWING
   SALES VOLUMES

- -  PROVEN TRACK RECORD OF SUCCESSFULLY INTEGRATING
   ACQUISITIONS

- -  INCREASED SIZE AND FINANCIAL CAPACITY PROVIDES SIGNIFICANT
   OPPORTUNITIES FOR FURTHER GROWTH



                     [LETTER TO NON-SAN ANTONIO EMPLOYEES]

Dear UDS Employee:

I am pleased to announce that we have accepted an exciting business opportunity.
We are  joining  forces  with Valero  Energy  Corporation  to create the premier
refining and marketing company in the nation.

Our combined  companies will have just under 2 million  barrels per day (BPD) of
refining capacity,  which will make us the largest independent  refining company
in the U.S. - second only to ExxonMobil in terms of total refining capacity. The
new  organization  will have  combined  revenues of $32 billion,  $10 billion in
total assets and a market capitalization of more than $6.1 billion.  Plus, based
on combined revenues,  it should be listed around 49th among the Fortune 500. By
combining  our  tremendous  assets,  we will  achieve  operational  and business
synergies  of at least $200  million  annually.  And,  the company will have the
highest earnings  leverage to strong refining  fundamentals,  but will also have
our strong retail operations to stabilize earnings.

It's not often that two major  Fortune  500  companies  from the same city could
accomplish  such an  exceptional  partnership.  By combining our resources  with
Valero,  we will have a stronger  market  presence,  become more  attractive  to
investors and be well  positioned for even further  growth and success.  And, by
pooling talents from both companies,  our combined organization will have a much
stronger management team and workforce.

Most importantly, the merger will provide several benefits and opportunities for
UDS employees:

o The opportunity to be a part of a bigger, stronger organization;  o Comparable
pay and benefits; o More opportunities for career advancement and development; o
And a commitment that virtually all employees will be retained.

It is important to stress that employees from both companies are critical to the
new organization's ongoing success. Valero has a great track record of acquiring
assets while  successfully  maintaining,  integrating and even adding employees.
And like UDS, Valero has a strong reputation as a company committed to the needs
of its employees and  communities,  ranking among Fortune  Magazine's  "100 Best
Companies to Work for in America" for the last two years.

Between  now and  closing,  we will  work  actively  with  Valero,  looking  for
opportunities  to  integrate  the  workforces,  and we'll  hold  open  positions
(wherever feasible) to create a large pool of jobs to fill after the transaction
closes.  Both  companies  will also  implement an  attractive,  voluntary  early
retirement program for interested employees.



We expect this  transaction to be complete  during the fourth quarter 2001. Over
the next few  weeks,  we'll be  providing  you  with  information,  updates  and
opportunities  to ask  questions.  Additionally,  the company's  Intranet  site,
Ultrarock, will feature continuous updates.

At 9 a.m.  today,  we invite you to view the live webcast of the analyst meeting
discussing this announcement via our corporate website at www.udscorp.com, click
on the "Investor Relations" section and select the "Live Webcast" link.

Bill Greehey  will be the  Chairman of the Board and CEO of the new Valero,  and
four UDS directors will join the new Valero board of directors.  I will continue
to run UDS until the closing,  and I will work with Bill on the  organization of
the new  company.  I will then  retire  from UDS and step down from the board of
directors.

Thank you, and I look  forward to working with you in the coming  months to make
this tremendous opportunity a reality.


Sincerely,
Jean Gaulin
Chairman, President and CEO
Ultramar Diamond Shamrock Corporation



                                       [Q and A]

Questions & Answers

1.       Why is the Company doing this?

A.       To become more attractive to investors in fulfilling  our  vision:
o        Being as attractive to investors as possible is the goal of any
         organization.
o        This transaction offers a significant premium to UDS shareholders and
         approximately a 40% participation in the combined
              companies.
o        Better liquidity for shareholders due to size.
o             Valero's stock is the best conduit to increase  shareholder  value
              due to its current price/earnings  multiplier,  which we expect to
              get even better.
o             Our industry's  environment  requires that we continue to grow and
              consolidate   operations  to  be  competitive  and  to  serve  our
              customers more efficiently.
o       Geographic diversity, size and growth opportunities in strategically
        important locations.

o             UDS brings with it a strong  marketing  and  logistics  group that
              Valero  does  not  currently  have  for   additional   operational
              diversity.

B.       Great for Employees:

o             Opportunity  to be a part of a larger  organization  that draws on
              the strength of both companies.  The new  organization  will be by
              far the largest independent  refining and marketing company in the
              country as well as the 2nd largest refiner in the country.

o             Virtually  all  employees  will be retained.  Any  duplication  in
              jobs/services  will  be  handled  through  hiring  freezes  and  a
              voluntary early retirement package.

o Many employees will find greater  opportunities  for diverse  experiences  and
advancement  o Similar  cultures  focused  on health,  safety  and  environment.
Continued commitment to the community.

C.       Great for Community:
o Headquarters stays in San Antonio; virtually all employees will be retained.
o Continued  commitment and leadership in the community as well as dedication to
charitable giving and community  service.  o Similar cultures focused on health,
safety and environment. Continued commitment to the community.

2.       How much will Valero pay for UDS?
     $27.50 in cash plus 0.614 VLO closing share price.

3.       When will the transaction be finalized?
     We expect the transaction  will be final by the 4th Quarter of 2001 - after
     the receipt of shareholder approvals from both companies and all regulatory
     approvals.

4.  Do you expect any roadblocks?
     We do not foresee any conditions that would prevent this  transaction  from
     proceeding. The Federal Trade Commission and various states will review the
     agreement,  but we are  optimistic  that their review will not have a major
     impact on the  transaction.  We believe  consumers  will  benefit  from our
     high-volume, low-price strategy and the independents will be better served.

5.       How many employees will lose their jobs?
     Virtually all employees will be retained.  The synergies of this merger are
     business and  operational in nature.  The management  team  responsible for
     developing an organizational plan and determining staffing  requirements is
     committed to using a hiring freeze and a voluntary early retirement program
     to  ensure  that  virtually  all  employees  will  have a place  in the new
     organization.


6.  How should we handle communication to the public, company representatives,
    and others? During the formation of the new  organization,  UDS is still
    competing with Valero and all information  exchanged must conform with legal
    requirements restricting  exchange of competitive  information.  Please
    consult with the legal  department  if  you  have  any  questions  about
    information  to be exchanged with Valero.  All  communication  with the
    public,  the media, or others is to be handled through the Public Relations,
    Investor  Relations, or the Legal Department.


7.   Will my benefits or pay structure change in anticipation of the new organi-
     zation?
     We anticipate it will take 6-9 months to complete formation of the combined
     organization.  There will be no changes in benefits or compensation related
     to this transaction prior to closing and comparable programs going forward.

8.  Will the new organization count my years of service with UDS?

Employees will receive credit for all prior service recognized by UDS.

9.  How will employee communications be handled?
     Throughout  the  transaction,  employees  will be kept informed  including:
     press releases,  employee meetings and communication via email. We will use
     the Leadership Council to help coordinate communications and get answers to
     your  questions.  Additionally,  a special  page  has/will be  developed on
     Ultrarock to allow  employees to read the latest news and submit  questions
     anonymously.

10.  Who can I talk to if I have questions and concerns?
      We  encourage  you to talk with  your  supervisor/manager,  or your  union
      representative,  about any  questions or concerns  you have.  You may also
      talk with your Human Resources Representative.  We will also set up a site
      to receive and respond to  questions.  Questions  and answers will also be
      published on the Intranet.

11.   Since both  organizations  have new headquarters  campuses in San Antonio,
      where will the corporate offices for the combined organization be located?
      We anticipate both offices will be used.

12.   You say virtually all employees  will be retained,  what  synergies is the
      combined company  anticipating that will make this such an attractive deal
      to the investors?

      While historically our acquisitions have relied on cost-cutting synergies,
      this  merger  creates  tremendous  value  for  the  shareholder  based  on
      liquidity,   multiplier   expansion  and  diversity   geographically   and
      operationally.   We  do  anticipate   tremendous  EBIT  improvements  from
      operations through our combined purchasing power and best practices.

13.      After such a profitable year, why aren't we buying Valero?

      The  organizations  have decided to combine in order to better  compete in
      our industry.  Valero's stock has  historically  been valued better by the
      investors and offered a better value to all shareholders.

14.       What happens to Jean and our Management Team?

      Jean will retire after closing and Bill Greehey will be the Chairman & CEO
      of the combined  organization.  We anticipate the Senior  Management  Team
      will be selected from both organizations.  But again,  employees from each
      organization  are on an equal footing and it is the combined  organization
      that will offer a better value to all  stakeholders  than each corporation
      individually.

15.      What is the name of the combined company?

      The Corporate name will be Valero Energy Corporation.

16.      Whose stock will be traded after this transaction is complete?

      The stock will be traded under the Valero symbol of VLO.

17.      When will employees be able to meet Bill Greehey?

     On May 17 and 18, Bill Greehey will be at UDS's corporate  headquarters and
     together he and Jean plan to meet with employees. He will be able to answer
     some of your  concerns,  but the details  have not yet all been  finalized.
     Visits to other sites will be made over the next few weeks.

18.      How will this merger affect me personally?

       Because we have just begun to work on the  details to the  merger,  we do
       not know your personal  situation yet. We will get  information to you as
       soon as possible.  There will be some changes due to the formation of the
       new organization,  and employees' jobs may be affected. Again, Valero has
       made a commitment to retain virtually all employees.

19.      Will there be a severance package/program available?
       Although  few, if any lay-offs are  expected,  to reassure  employees and
       minimize anxiety,  an enhanced  severance program has been developed.  It
       will consist of severance pay and benefits,  outplacement assistance, and
       will provide for access to the employee  assistance  plan (EAP) and other
       stress  management aids. The EAP will also be available for employees and
       their families. Additional details will be provided shortly.

20.      How can we possibly take on another major task with all of the other
        projects we have going?

       Our focus must be to continue to perform our job well. Please try to stay
       focused  on your  job.  Although  from  time to time you may be asked for
       information or to otherwise  participate in the transition process, it is
       unlikely that such  requests  will require a significant  portion of most
       employees' time.

21.      Are we still going to receive our merit increases?

       Business will  continue as normal.  We anticipate no change in the annual
review process.

22.      When will we know whether we get to keep our jobs or if they will be
                eliminated?

       Again,  Valero is committed to retaining  virtually all employees.  We do
       not know the exact date the  closing  will  occur,  although we expect it
       will be in the 4th  quarter.  We have not yet  estimated  the length of a
       post-closing transition period.  However,  organizational changes will be
       announced in a timely fashion.

23.      How are we going to deal with this hiring freeze?

       We will  continue to monitor the  situation  and we will  continue to use
outside resources as necessary.

24.      What will happen with the PPP, AIP and other bonus programs?

       These  programs  will  continue.  The goal is to continue with our profit
       improvement  strategies.  If we meet the  goals  established,  the  bonus
       programs will pay out as scheduled.

25.      Will this transaction have any effect on the Canadian employees and
               operations?

       This  transaction  will  not  have a  direct  impact  on  UDS's  Canadian
       operations.  However,  it will make them part of a bigger  company with a
       stronger  parent,  a stronger  balance sheet and more  opportunities  for
       employees.



26.      Will there be an opportunity for promotions and/or increased pay/
        benefits because of the additional workload/responsibility?
       It's too early to predict what precise opportunities will be available.

27.      What are your plans regarding brands?
     The strongest brand will be selected on a  market-by-market  basis with the
     intention of optimizing  the  historical  strengths and loyal customer base
     while minimizing rebranding costs.

28.      If my job is eliminated, will I be considered for other jobs ?
       Yes.

29.      How will I know what other jobs may be available?
     Jobs will be posted and communicated as in the past.

30.      What are the metrics of the new Valero operations?
       It will operate 13  refineries  in North  America  with a total  combined
       throughput  capacity  of  over  1.8  million  barrels  per  day.  The new
       organization  will operate over 5000 retail  outlets in 17 states and six
       provinces in eastern  Canada.  It also will operate a crude and petroleum
       products   transportation  system  consisting  of  over  4,600  miles  of
       pipelines,  a unit train, and a number of  strategically  located product
       terminals.

                       [LETTER TO SAN ANTONIO EMPLOYEES]

Dear UDS Employee:

I am pleased to announce that we have accepted an exciting business opportunity.
We are  joining  forces  with Valero  Energy  Corporation  to create the premier
refining and marketing company in the nation.

Our combined  companies will have just under 2 million  barrels per day (BPD) of
refining capacity,  which will make us the largest independent  refining company
in the U.S. - second only to ExxonMobil in terms of total refining capacity. The
new  organization  will have  combined  revenues of $32 billion,  $10 billion in
total assets and a market capitalization of more than $6.1 billion.  Plus, based
on combined revenues,  it should be listed around 49th among the Fortune 500. By
combining  our  tremendous  assets,  we will  achieve  operational  and business
synergies  of at least $200  million  annually.  And,  the company will have the
highest earnings  leverage to strong refining  fundamentals,  but will also have
our strong retail operations to stabilize earnings.

It's not often that two major  Fortune  500  companies  from the same city could
accomplish  such an  exceptional  partnership.  By combining our resources  with
Valero,  we will have a stronger  market  presence,  become more  attractive  to
investors and be well  positioned for even further  growth and success.  And, by
pooling talents from both companies,  our combined organization will have a much
stronger management team and workforce.

Most importantly, the merger will provide several benefits and opportunities for
UDS employees:

o The opportunity to be a part of a bigger, stronger organization;  o Comparable
pay and benefits; o More opportunities for career advancement and development; o
And a commitment that virtually all employees will be retained.


It is important to stress that employees from both companies are critical to the
new organization's ongoing success. Valero has a great track record of acquiring
assets while  successfully  maintaining,  integrating and even adding employees.
And like UDS, Valero has a strong reputation as a company committed to the needs
of its employees and  communities,  ranking among Fortune  Magazine's  "100 Best
Companies to Work for in America" for the last two years.

Between  now and  closing,  we will  work  actively  with  Valero,  looking  for
opportunities  to  integrate  the  workforces,  and we'll  hold  open  positions
(wherever feasible) to create a large pool of jobs to fill after the transaction
closes.  Both  companies  will also  implement an  attractive,  voluntary  early
retirement program for interested employees.



We expect this  transaction to be complete  during the fourth quarter 2001. Over
the next few  weeks,  we'll be  providing  you  with  information,  updates  and
opportunities  to ask  questions.  Additionally,  the company's  Intranet  site,
Ultrarock, will feature continuous updates.

Today, we invite you to participate in several events as follows:

1.  Meeting with company executives at 8 a.m. OR 10:30 a.m. in the cafeteria;
2.  At 9 a.m.,  viewing the live webcast of the analyst meeting  discussing this
    announcement  via our  corporate  website at  www.udscorp.com,  click on the
    "Investor  Relations"  section and select the "Live Webcast" link. This will
    also be broadcast in the cafeteria.

Bill Greehey  will be the  Chairman of the Board and CEO of the new Valero,  and
four UDS directors will join the new Valero board of directors.  I will continue
to run UDS until the closing,  and I will work with Bill on the  organization of
the new  company.  I will then  retire  from UDS and step down from the board of
directors.

Thank you, and I look  forward to working with you in the coming  months to make
this tremendous opportunity a reality.


Sincerely,
Jean Gaulin
Chairman, President and CEO
Ultramar Diamond Shamrock