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                                                                     EXHIBIT 4.9


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                SERIES A CAPITAL SECURITIES GUARANTEE AGREEMENT


                                NGC CORPORATION

                           Dated as of May 28, 1997


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                               TABLE OF CONTENTS

                                                                            Page
                    ARTICLE I DEFINITIONS AND INTERPRETATION

SECTION 1.1   Definitions and Interpretation................................  2

                        ARTICLE II TRUST INDENTURE ACT

SECTION 2.1   Trust Indenture Act; Application..............................  6
SECTION 2.2   Lists of Holders of Securities................................  6
SECTION 2.3   Reports by the Capital Securities Guarantee Trustee...........  6
SECTION 2.4   Periodic Reports to Capital Securities Guarantee Trustee......  7
SECTION 2.5   Evidence of Compliance with Conditions Precedent..............  7
SECTION 2.6   Events of Default; Waiver.....................................  7
SECTION 2.7   Event of Default; Notice......................................  7
SECTION 2.8   Conflicting Interests.........................................  8
SECTION 2.9   Capital Securities Guarantee Trustee May File
              Proofs of Claim...............................................  8

                                 ARTICLE III
       POWERS, DUTIES AND RIGHTS OFCAPITAL SECURITIES GUARANTEE TRUSTEE

SECTION 3.1   Powers and Duties of the Capital Securities Guarantee 
              Trustee.......................................................  8
SECTION 3.2   Certain Rights of Capital Securities Guarantee Trustee........ 10
SECTION 3.3   Not Responsible for Recitals or Issuance of Series A Capital
              Securities Guarantee.......................................... 12

                ARTICLE IV CAPITAL SECURITIES GUARANTEE TRUSTEE

SECTION 4.1    Capital Securities Guarantee Trustee; Eligibility............ 12
SECTION 4.2    Appointment, Removal and Resignation of Capital Securities
               Guarantee Trustee............................................ 13

                              ARTICLE V GUARANTEE

SECTION 5.1   Guarantee..................................................... 14
SECTION 5.2   Waiver of Notice and Demand................................... 14
SECTION 5.3   Obligations Not Affected...................................... 14
SECTION 5.4   Rights of Holders............................................. 15
SECTION 5.5   Guarantee of Payment.......................................... 16
SECTION 5.6   Subrogation................................................... 16
SECTION 5.7   Independent Obligations....................................... 16

             ARTICLE VI LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 6.1   Limitation of Transactions.................................... 17
SECTION 6.2   Ranking....................................................... 18

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                            ARTICLE VII TERMINATION

SECTION 7.1   Termination................................................... 18

                         ARTICLE VIII INDEMNIFICATION

SECTION 8.1   Exculpation................................................... 18
SECTION 8.2   Indemnification............................................... 19

                           ARTICLE IX MISCELLANEOUS

SECTION 9.1   Successors and Assigns........................................ 19
SECTION 9.2   Amendments.................................................... 19
SECTION 9.3   Notices....................................................... 20
SECTION 9.4   Exchange Offer................................................ 21
SECTION 9.5   Benefit....................................................... 21
SECTION 9.6   GOVERNING LAW................................................. 21
SECTION 9.7   Limited Liability............................................. 21

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                               SERIES A CAPITAL
                        SECURITIES GUARANTEE AGREEMENT


          This GUARANTEE AGREEMENT (the "Series A Capital Securities
Guarantee"), dated as of May 28, 1997, is executed and delivered by NGC
Corporation, a Delaware corporation (the "Guarantor"), and The First National
Bank of Chicago, a national banking association as trustee (the "Capital
Securities Guarantee Trustee"), for the benefit of the Holders (as defined
herein) from time to time of the Series A Capital Securities (as defined herein)
of NGC Corporation Capital Trust I, a Delaware statutory business trust (the
"Issuer").

          WHEREAS, pursuant to the Declaration (as defined herein) the Issuer is
issuing on the date hereof $200,000,000 aggregate liquidation amount of its
Series A 8.316% Capital Securities (collectively the "Series A Capital
Securities") and, in connection with an Exchange Offer (as defined in the
Declaration), liquidation amount $1,000 per Capital Security, has agreed to
execute and deliver the Series B Capital Securities Guarantee (as defined in the
Declaration) for the benefit of holders of the Series B Capital Securities (as
defined in the Declaration).

          WHEREAS, as incentive for the Holders to purchase the Series A Capital
Securities, the Guarantor desires irrevocably and unconditionally to agree, to
the extent set forth in this Series A Capital Securities Guarantee, to pay to
the Holders the Guarantee Payments (as defined below) and to make certain other
payments on the terms and conditions set forth herein.

          WHEREAS, the Guarantor is executing and delivering a guarantee
agreement (the "Common Securities Guarantee"), with substantially identical
terms to this Series A Capital Securities Guarantee, for the benefit of the
holders of the Common Securities (as defined herein), 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 Guarantee Payments under
the Common Securities Guarantee are subordinated, to the extent and in the
manner set forth in the Common Securities Guarantee, to the rights of holders of
Series A Capital Securities and the Series B Capital Securities to receive
Guarantee Payments under this Series A Capital Securities Guarantee and the
Series B Capital Securities Guarantee, as the case may be.

          NOW, THEREFORE, in consideration of the purchase by each Holder, which
purchase the Guarantor hereby acknowledges shall benefit the Guarantor, the
Guarantor executes and delivers this Series A Capital Securities Guarantee for
the benefit of the Holders.


                                   ARTICLE I
                        DEFINITIONS AND INTERPRETATION

SECTION 1.1   Definitions and Interpretation
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          In this Series A Capital Securities Guarantee, unless the context
otherwise requires:

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

          (2) terms defined in the Declaration as at the date of execution of
     this Series A Capital Securities Guarantee have the same meaning when used
     in this Series A Capital Securities Guarantee unless otherwise defined in
     this Series A Capital Securities Guarantee;

          (3) a term defined anywhere in this Series A Capital Securities
     Guarantee has the same meaning throughout;

          (4) all references to "the Series A Capital Securities Guarantee" or
     "this Series A Capital Securities Guarantee" are to this Series A Capital
     Securities Guarantee as modified, supplemented or amended from time to
     time;

          (5) all references in this Series A Capital Securities Guarantee to
     Articles and Sections are to Articles and Sections of this Series A Capital
     Securities Guarantee, unless otherwise specified;

          (6) a term defined in the Trust Indenture Act has the same meaning
     when used in this Series A Capital Securities Guarantee, unless otherwise
     defined in this Series A Capital Securities Guarantee or unless the context
     otherwise requires; and

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

          "Affiliate" has the same meaning as given to that term in Rule 405
under the Securities Act of 1933, as amended, or any successor rule thereunder.

          "Business Day" means any day other than a Saturday or a Sunday, or a
day on which banking institutions in Chicago, Illinois or Houston, Texas are
authorized or required by law or executive order to close.

          "Capital Securities Guarantee Trustee" means The First National Bank
of Chicago, a national banking association, until a Successor Capital Securities
Guarantee Trustee has been appointed and has accepted such appointment pursuant
to the terms of this Series A Capital Securities Guarantee and thereafter means
each such Successor Capital Securities Guarantee Trustee.

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          "Common Securities" means the securities representing common undivided
beneficial interests in the assets of the Issuer.

          "Corporate Trust Office" means the office of the Capital Securities
Guarantee Trustee at which the corporate trust business of the Capital
Securities Guarantee Trustee shall, at any particular time, be principally
administered, which office at the date of execution of this Agreement is located
at One First National Plaza, Suite 0126, Chicago, Illinois  60670.

          "Covered Person" means any Holder or beneficial owner of Series A
Capital Securities.

          "Debentures" means the series of subordinated debt securities of the
Guarantor designated the Series A 8.316% Subordinated Deferrable Interest
Debentures due June 1, 2027 held by the Property Trustee (as defined in the
Declaration) of the Issuer.

          "Declaration" means the Amended and Restated Declaration of Trust,
dated as of May 28, 1997 as amended, modified or supplemented from time to time,
among the trustees of the Issuer named therein, the Guarantor, as sponsor, and
the holders from time to time of undivided beneficial ownership interests in the
assets of the Issuer.

          "Event of Default" means a default by the Guarantor on any of its
payment or other obligations under this Series A Capital Securities Guarantee,
provided, however, that with respect to a default other than a default in
payment of any Guarantee Payment, the Guarantor shall have received 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 Series A Capital Securities, to the
extent not paid or made by the Issuer:  (i) any accumulated and unpaid
Distributions (as defined in the Declaration) that are required to be paid on
such Series A Capital Securities to the extent the Issuer has funds on hand
legally available therefor at such time, (ii) the redemption price, including
all accumulated and unpaid Distributions to the date of redemption (the
"Redemption Price") to the extent the Issuer has funds on hand legally available
therefor at such time, with respect to any Series A Capital Securities called
for redemption by the Issuer, and (iii) upon a voluntary or involuntary
dissolution, winding-up or liquidation of the Issuer (other than in connection
with the distribution of Debentures to the Holders in exchange for Series A
Capital Securities as provided in the Declaration), the lesser of (a) the
aggregate of the liquidation amount and all accumulated and unpaid Distributions
on the Series A Capital Securities to the date of payment, to the extent the
Issuer has funds on hand legally available therefor, and (b) the amount of
assets of the Issuer remaining available for distribution to Holders in
liquidation of the Issuer after satisfaction of liabilities to creditors of the
Issuer as required by applicable law.  If an Event of Default has occurred and
is continuing, no Guarantee Payments under the Common Securities Guarantee with
respect to the Common Securities or any guarantee payment 

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under any Other Common Securities Guarantees shall be made until the Holders
shall be paid in full the Guarantee Payments to which they are entitled under
this Series A Capital Securities Guarantee.

          "Holder" shall mean any holder, as registered on the books and records
of the Issuer, of any Series A Capital Securities; provided, however, that, in
determining whether the holders of the requisite percentage of Series A Capital
Securities have given any request, notice, consent or waiver hereunder, "Holder"
shall not include the Guarantor or any Person known to a Responsible Officer of
the Capital Securities Guarantee Trustee to be an Affiliate of the Guarantor and
provided further, that in determining whether the Holders of the requisite
liquidation amount of Series A Capital Securities have voted on any matter
provided for in the Guarantee, then for the purpose of such determination only
(and not for any other purposes hereunder), if the Series A Capital Securities
remain in the form of one or more Global Certificates (as defined in the
Declaration), the term "Holders" shall mean the holder of the Global Certificate
acting at the direction of the Capital Security Beneficial Owners (as defined in
the Declaration).

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

          "Indenture" means the Indenture dated as of May 28, 1997, among the
Guarantor (the "Debenture Issuer") and The First National Bank of Chicago, as
trustee (the "Indenture Trustee"), and any indenture supplemental thereto,
pursuant to which the Debentures are to be issued to the Property Trustee (as
defined in the Declaration) of the Issuer.

          "Indenture Event of Default" shall mean any event specified in 
Section 5.01 of the Indenture.

          "Majority in liquidation amount of the Series A Capital Securities"
means, except as provided by the Declaration or by the Trust Indenture Act, a
vote by Holder(s) of more than 50% of the aggregate liquidation amount of all
outstanding Series A Capital Securities voting separately as a class.  In
determining whether the Holders of the requisite amount of the Series A Capital
Securities have voted, Series A Capital Securities which are owned by the
Guarantor or any Affiliate of the Guarantor shall be disregarded for the purpose
of any such determination period.

          "Officers' Certificate" means, with respect to the Guarantor, a
certificate signed by any of the Chairman of the Board, a Vice Chairman of the
Board, the Chief Executive Officer, the President or a Vice President and by the
Treasurer, an Assistant Treasurer, the Secretary or an Assistant Secretary of
the Guarantor.  Any Officers' Certificate delivered with respect to compliance
with a condition or covenant provided for in this Series A Capital Securities
Guarantee (other than pursuant to Section 314(d)(4) of the Trust Indenture Act)
shall include:

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          (a) a statement that each officer signing the Officers' Certificate
     has read the covenant or condition and the definitions relating thereto;

          (b) a brief statement of the nature and scope of the examination or
     investigation undertaken by each officer on behalf of such Person in
     rendering the Officers' Certificates.

          (c) a statement that each such officer has made such examination or
     investigation as, in such officer's opinion, is necessary to enable such
     officer to express an informed opinion as to whether or not such covenant
     or condition has been complied with; and

          (d) a statement as to whether, in the opinion of each such officer,
     such condition or covenant has been complied with.

          "Other Common Securities Guarantees" shall have the same meaning as
"Other Guarantees" as defined in the Common Securities Guarantee.

          "Other Debentures" means all junior subordinated debentures issued by
the Guarantor from time to time and sold to trusts to be established by the
Guarantor (if any), in each case similar to the Issuer.

          "Other Guarantees" means all guarantees to be issued by the Guarantor
with respect to capital securities (if any) similar to the Series A Capital
Securities issued by other trusts to be established by the Guarantor (if any),
in each case similar to the Issuer.

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

          "Registration Rights Agreement"  means the Registration Rights
Agreement, dated as of May 28, 1997, by and among the Guarantor, the Issuer and
the initial purchasers named therein as such agreement may be amended, modified
or supplemented from time to time.

          "Responsible Officer" means, with respect to the Capital Securities
Guarantee Trustee, any officer within the Corporate Trust Office of the Capital
Securities Guarantee Trustee with direct responsibility for the administration
of this Series A Capital Securities Guarantee 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 Capital Securities Guarantee Trustee" means a successor
Capital Securities Guarantee Trustee possessing the qualifications to act as
Capital Securities Guarantee Trustee under Section 4.1.

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          "Trust Indenture Act" means the Trust Indenture Act of 1939, as
amended from time to time, or any successor legislation.

          "Trust Securities" means the Common Securities and the Series A
Capital Securities and Series B Capital Securities, collectively.


                                  ARTICLE II
                              TRUST INDENTURE ACT

SECTION 2.1    Trust Indenture Act; Application

               (a) This Series A Capital Securities Guarantee is subject to the
provisions of the Trust Indenture Act that are required to be part of this
Series A Capital 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 Series A
Capital 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 of Securities

               (a) The Guarantor shall provide the Capital Securities Guarantee
Trustee (unless the Capital Securities Guarantee Trustee is otherwise the
registrar of the Capital Securities) with a list, in such form as the Capital
Securities Guarantee Trustee may reasonably require, of the names and addresses
of the Holders ("List of Holders") as of such date, (i) within one Business Day
after May 15 and November 15 of each year, 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
Capital Securities Guarantee Trustee, provided, that the Guarantor shall not be
obligated to provide such List of Holders at any time that the Guarantor
certifies in writing to the Capital Securities Guarantee Trustee that the List
of Holders does not differ from the most recent List of Holders given to the
Capital Securities Guarantee Trustee by the Guarantor.  The Capital Securities
Guarantee Trustee may destroy any List of Holders previously given to it on
receipt of a new List of Holders.

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

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<PAGE>
 
SECTION 2.3    Reports by the Capital Securities Guarantee Trustee

               Within 60 days after May 15 of each year, commencing May 15,
1998, the Capital Securities 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 Capital Securities Guarantee Trustee shall also comply with the other
requirements of Section 313 of the Trust Indenture Act.

SECTION 2.4    Periodic Reports to Capital Securities Guarantee Trustee

               The Guarantor shall provide to the Capital Securities Guarantee
Trustee such documents, reports and information as 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; provided that such compliance certificate shall
be delivered on or before 120 days after the end of each fiscal year of the
Guarantor.  Delivery of such reports, information and documents to the Capital
Securities Guarantee Trustee is for informational purposes only and the Capital
Securities Guarantee Trustee's receipt of such shall not constitute constructive
notice of any information contained therein or determinable from information
contained therein, including the Guarantor's compliance with any of its
covenants hereunder (as to which the Capital Securities Guarantee Trustee is
entitled to rely exclusively on Officers' Certificates).

SECTION 2.5    Evidence of Compliance with Conditions Precedent

               The Guarantor shall provide to the Capital Securities Guarantee
Trustee such evidence of compliance with any conditions precedent, if any,
provided for in this Series A Capital 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 a Majority in liquidation amount of Series A
Capital Securities may, by vote, on behalf of all Holders, 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 this Series A Capital 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 Capital Securities Guarantee Trustee shall, within 90
days after the occurrence of an Event of Default with respect to this Capital
Securities Guarantee actually known to a Responsible Officer, mail by first
class postage prepaid, to all Holders, notices of all such Events 

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of Default, unless such defaults have been cured before the giving of such
notice, provided, that, except in the case of default in the payment of any
Guarantee Payment, the Capital Securities Guarantee Trustee shall be protected
in withholding such notice if and so long as the board of directors, the
executive committee, or a trust committee of directors and/or a Responsible
Officer in good faith determines that the withholding of such notice is in the
interests of the Holders.

              (b)  The Capital Securities Guarantee Trustee shall not be deemed
to have knowledge of any Event of Default unless the Capital Securities
Guarantee Trustee shall have received written notice from the Guarantor, or a
Responsible Officer charged with the administration of this Series A Capital
Securities Guarantee shall have obtained actual knowledge, of such Event of
Default.

SECTION 2.8   Conflicting Interests

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


SECTION 2.9   Capital Securities Guarantee Trustee May File Proofs of Claim

              Upon the occurrence of an Event of Default, the Capital Securities
Guarantee Trustee is hereby authorized to (a) recover judgment, in its own name
and as trustee of an express trust, against the Guarantor for the whole amount
of any Guarantee, payments remaining unpaid and (b) file such proofs of claim
and other papers or documents as may be necessary or advisable in order to have
its claims and those of the Holders of the Series A Capital Securities allowed
in any judicial procedures relative to the Guarantor, its creditors or its
property.

                                  ARTICLE III
                         POWERS, DUTIES AND RIGHTS OF
                     CAPITAL SECURITIES GUARANTEE TRUSTEE

SECTION 3.1   Powers and Duties of the Capital Securities Guarantee Trustee

              (a)  This Series A Capital Securities Guarantee shall be held by
the Capital Securities Guarantee Trustee for the benefit of the Holders, and the
Capital Securities Guarantee Trustee shall not transfer this Series A Capital
Securities Guarantee to any Person except a Holder exercising his or her rights
pursuant to Section 5.4(b) or to a Successor Capital Securities Guarantee
Trustee on acceptance by such Successor Capital Securities Guarantee Trustee of
its appointment to act as Successor Capital Securities Guarantee Trustee. The
right, title and interest of the Capital Securities Guarantee Trustee shall
automatically vest in any Successor Capital Securities Guarantee Trustee, and
such vesting and succession of title shall be effective whether or not
conveyancing 

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documents have been executed and delivered pursuant to the appointment of such
Successor Capital Securities Guarantee Trustee.

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

              (c)  The Capital Securities 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 Series A Capital Securities Guarantee, and no
implied covenants shall be read into this Series A Capital Securities Guarantee
against the Series A Capital Securities 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, the Capital Securities Guarantee
Trustee shall exercise such of the rights and powers vested in it by this Series
A Capital Securities Guarantee, and 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 Series A Capital Securities Guarantee
shall be construed to relieve the Capital Securities 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:

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

                   (B)  in the absence of bad faith on the part of the Capital
              Securities Guarantee Trustee, the Capital Securities Guarantee
              Trustee may conclusively rely, as to the truth of the statements
              and the correctness of the opinions expressed therein, upon any
              certificates or opinions furnished to the Capital Securities
              Guarantee Trustee and conforming to the requirements of this
              Series A Capital 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 Capital Securities
              Guarantee Trustee, the Capital Securities Guarantee Trustee shall
              be under a duty to examine the same to 

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              determine whether or not they conform to the requirements of this
              Series A Capital Securities Guarantee;

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

              (iii) the Capital Securities 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 a Majority in
     liquidation amount of the Series A Capital Securities relating to the time,
     method and place of conducting any proceeding for any remedy available to
     the Capital Securities Guarantee Trustee, or exercising any trust or power
     conferred upon the Capital Securities Guarantee Trustee under this Series A
     Capital Securities Guarantee; and

              (iv)  no provision of this Series A Capital Securities Guarantee
     shall require the Capital Securities 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 rights or
     powers, if the Capital Securities 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 Series A Capital
     Securities Guarantee or indemnity, reasonably satisfactory to the Capital
     Securities Guarantee Trustee, against such risk or liability is not
     reasonably assured to it.

SECTION 3.2   Certain Rights of Capital Securities Guarantee Trustee

              (a) Subject to the provisions of Section 3.1:

              (i)   The Capital Securities 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, note, other
     evidence of indebtedness or other paper 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
     Series A Capital Securities Guarantee may be sufficiently evidenced by an
     Officers' Certificate.

              (iii) Whenever, in the administration of this Series A Capital
     Securities Guarantee, the Capital Securities Guarantee Trustee shall deem
     it desirable that a matter be proved or established before taking,
     suffering or omitting any action hereunder, the Capital Securities
     Guarantee Trustee (unless other evidence is herein specifically prescribed)
     may, in the 

                                       10
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     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 Capital Securities Guarantee Trustee shall have no duty
     to see to any recording, filing or registration of any instrument (or any
     rerecording, refiling or registration thereof).

              (v)    The Capital Securities 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 Capital Securities Guarantee Trustee
     shall have the right at any time to seek instructions concerning the
     administration of this Series A Capital Securities Guarantee from any court
     of competent jurisdiction.

              (vi)   The Capital Securities Guarantee Trustee shall be under no
     obligation to exercise any of the rights or powers vested in it by this
     Series A Capital Securities Guarantee at the request or direction of any
     Holder, unless such Holder shall have provided to the Capital Securities
     Guarantee Trustee such security and indemnity, reasonably satisfactory to
     the Capital Securities Guarantee Trustee, against the costs, expenses
     (including attorneys' fees and expenses and the expenses of the Capital
     Securities 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
     Capital Securities Guarantee Trustee; provided that, nothing contained in
     this Section 3.2(a)(vi) shall be taken to relieve the Capital Securities
     Guarantee Trustee, upon the occurrence of an Event of Default, of its
     obligation to exercise the rights and powers vested in it by this Series A
     Capital Securities Guarantee.

              (vii)  The Capital Securities 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 Capital Securities
     Guarantee Trustee, in its discretion, may make such further inquiry or
     investigation into such facts or matters as it may see fit.

              (viii) The Capital Securities 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 Capital Securities 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.

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              (ix)  Any action taken by the Capital Securities Guarantee Trustee
     or its agents hereunder shall bind the Holders, and the signature of the
     Capital Securities 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 Capital Securities
     Guarantee Trustee to so act or as to its compliance with any of the terms
     and provisions of this Series A Capital Securities Guarantee, both of which
     shall be conclusively evidenced by the Capital Securities Guarantee
     Trustee's or its agent's taking such action.

              (x)   Whenever in the administration of this Series A Capital
     Securities Guarantee the Capital Securities Guarantee Trustee shall deem it
     desirable to receive instructions with respect to enforcing any remedy or
     right or taking any other action hereunder, the Capital Securities
     Guarantee Trustee (i) may request instructions from the Holders of a
     Majority in liquidation amount of the Series A Capital 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 Capital Securities Guarantee Trustee shall not be liable
     for any action taken, suffered, or omitted to be taken by it in good faith,
     without negligence, and reasonably believed by it to be authorized or
     within the discretion or rights or powers conferred upon it by this Series
     A Capital Securities Guarantee.

              (b)   No provision of this Series A Capital Securities Guarantee
shall be deemed to impose any duty or obligation on the Capital Securities
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 Capital Securities 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 Capital Securities Guarantee
Trustee shall be construed to be a duty.

SECTION 3.3   Not Responsible for Recitals or Issuance of Series A Capital
              Securities Guarantee

              The recitals contained in this Series A Capital Securities
Guarantee shall be taken as the statements of the Guarantor, and the Capital
Securities Guarantee Trustee does not assume any responsibility for their
correctness. The Capital Securities Guarantee Trustee makes no representation as
to the validity or sufficiency of this Series A Capital Securities Guarantee.


                                  ARTICLE IV
                     CAPITAL SECURITIES GUARANTEE TRUSTEE

SECTION 4.1   Capital Securities Guarantee Trustee; Eligibility

                                       12
<PAGE>
 
              (a)   There shall at all times be a Capital Securities 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 Capital Securities Guarantee Trustee
shall cease to be eligible to so act under Section 4.1(a), the Capital
Securities Guarantee Trustee shall immediately resign in the manner and with the
effect set out in Section 4.2(c).

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

 SECTION 4.2  Appointment, Removal and Resignation of Capital Securities
              Guarantee Trustee

              (a)  Subject to Section 4.2(b), the Capital Securities Guarantee
Trustee may be appointed or removed without cause at any time by the Guarantor
except during an Event of Default.

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

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

                                       13
<PAGE>
 
instrument in writing executed by such Successor Capital Securities Guarantee
Trustee and delivered to the Guarantor and the resigning Capital Securities
Guarantee Trustee.

              (d)   If no Successor Capital Securities 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 removal or resignation, the
Capital Securities Guarantee Trustee resigning or being removed may petition any
court of competent jurisdiction for appointment of a Successor Capital
Securities Guarantee Trustee. Such court may thereupon, after prescribing such
notice, if any, as it may deem proper, appoint a Successor Capital Securities
Guarantee Trustee.

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

              (f)   Upon termination of this Series A Capital Securities
Guarantee or removal or resignation of the Capital Securities Guarantee Trustee
pursuant to this Section 4.2, the Guarantor shall pay to the Capital Securities
Guarantee Trustee all amounts due to the Capital Securities Guarantee Trustee
accrued to the date of such termination, removal or resignation.


                                   ARTICLE V
                                   GUARANTEE

SECTION 5.1   Guarantee

              The Guarantor irrevocably 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 Series A
Capital 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 Subordinated Debentures and the Guarantor shall not be
obligated hereunder to make any Guarantee Payment during any extended Interest
Payment Period (as defined in the Indenture) with respect to the Distributions
(as defined in the Declaration) on the Series A Capital Securities.

                                       14
<PAGE>
 
SECTION 5.3   Obligations Not Affected

              The obligations, covenants, agreements and duties of the Guarantor
under this Series A Capital Securities Guarantee shall be absolute and
unconditional and shall remain in full force and effect until the entire
liquidation amount of all outstanding Series A Capital Securities shall have
been paid and such obligation 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 relat ing to the Series A Capital 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 terms of the Series A Capital Securities or the
extension of time for the performance of any other obligation under, arising out
of, or in connection with, the Series A Capital 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 permitted by the Indenture);

              (c)   any failure, omission, delay or lack of diligence on the
part of the Property Trustee or the Holders to enforce, assert or exercise any
right, privilege, power or remedy conferred on the Property Trustee or the
Holders pursuant to the terms of the Series A Capital 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 Series A
Capital Securities;

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

              (g)   the consummation of the Exchange Offer; or

              (h)   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 with
respect to the Guarantee Payments shall be absolute and unconditional under any
and all circumstances.

                                       15
<PAGE>
 
          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)   The Holders of a Majority in liquidation amount of the
Series A Capital Securities have the right to direct the time, method and place
of conducting any proceeding for any remedy available to the Capital Securities
Guarantee Trustee in respect of this Series A Capital Securities Guarantee or
exercising any trust or power conferred upon the Capital Securities Guarantee
Trustee under this Series A Capital Securities Guarantee.

              (b)   If the Capital Securities Guarantee Trustee fails to enforce
such Series A Capital Securities Guarantee, any Holder may institute a legal
proceeding directly against the Guarantor to enforce its rights under this
Series A Capital Securities Guarantee, without first instituting a legal
proceeding against the Issuer, the Capital Securities Guarantee Trustee or any
other person or entity. Notwithstanding the foregoing, if the Guarantor has
failed to make a Guarantee Payment, a Holder may directly institute a proceeding
in such Holder's own name against the Guarantor for enforcement of the Capital
Securities Guarantee for such payment. The Guarantor waives any right or remedy
to require that any action be brought first against the Issuer or any other
person or entity before proceeding directly against the Guarantor.

SECTION 5.5   Guarantee of Payment

              This Series A Capital 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 against the Issuer in respect of any amounts paid to such Holders by the
Guarantor under this Series A Capital 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 Series A Capital
Securities Guarantee, if, at the time of any such payment, any amounts are due
and unpaid under this Series A Capital 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.

                                       16
<PAGE>
 
SECTION 5.7   Independent Obligations

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


                                  ARTICLE VI
                   LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 6.1   Limitation of Transactions

              So long as any Series A Capital Securities remain outstanding, the
Guarantor shall not, and shall not permit any subsidiary of the Guarantor to,
(i) declare or pay any dividends or distributions on, or redeem, purchase,
acquire, or make a liquidation payment with respect to, any of the Guarantor's
capital stock (which includes common and preferred stock)(other than (a)
dividends or distributions in shares of, or options, warrants, rights to
subscribe for or purchase shares of, common stock of the Guarantor, (b) any
declaration of a dividend in connection with the implementation of a
stockholders' rights plan, or the issuance of stock under any such plan in the
future, or the redemption or repurchase of any such rights pursuant thereto, (c)
as a result of a reclassification of the Guarantor's capital stock or the
exchange or the conversion of one class or series of the Guarantor's capital
stock for another class or series of the Guarantor's capital stock, (d) the
purchase of fractional interests in shares of the Guarantor's capital stock
pursuant to the conversion or exchange provisions of such capital stock or the
security being converted or exchanged, and (e) purchases of the Guarantor's
common stock related to the issuance of the Guarantor's common stock or rights
under any of the Guarantor's benefit plans for its directors, officers,
employees or any of the Guarantor's dividend reinvestment plans), (ii) make any
payment of principal of, or premium, if any, or interest on or repay, repurchase
or redeem any debt securities of the Guarantor (including any Other Debentures)
that rank pari passu with or junior in right of payment to the Debentures or
(iii) make any guarantee payments with respect to any guarantee (other than
payments under the Series A Capital Securities Guarantee or the Series B Capital
Securities Guarantee) by the Guarantor of the debt securities of any subsidiary
of the Guarantor (including Other Guarantees) if such guarantee ranks pari passu
with or junior in right of payment to the Debentures if at such time (1) there
shall have occurred any event of which the Guarantor has actual knowledge that
(a) with the giving of notice or the lapse of time, or both, would constitute an
Indenture Event of Default and (b) in respect of which the Guarantor shall not
have taken reasonable steps to cure, (2) an Indenture Event of Default shall
have occurred and be continuing, (3) if such Debentures are held by the Property
Trustee, the Guarantor shall be in default with respect to its payment of any
obligations under this Series A Capital Securities Guarantee or (4) the
Guarantor shall have given notice of its election of the exercise of its right
to extend the interest payment period pursuant to Section 16.01 of the Indenture
or such extension period, or any such extension shall have commenced and be
continuing.

                                       17
<PAGE>
 
     Notwithstanding the foregoing, if an Event of Default has occurred and is
continuing under the Declaration, the rights of the holders of the Common
Securities to receive Guarantee Payments under the Common Securities Guarantee
or any Other Common Securities Guarantee shall be subordinated to the rights of
the holders of the Series A Capital Securities, the Series B Capital Securities
or the Private Exchange Capital Securities to receive payments of all amounts
due and owing to such holders under this Series A Capital Securities Guarantee,
any guarantee relating to the Series B Capital Securities or the Private
Exchange Capital Securities or any Other Guarantee.

SECTION 6.2   Ranking

              This Series A Capital Securities Guarantee will constitute an
unsecured obligation of the Guarantor and will rank (i) subordinate and junior
in right of payment to Senior Indebtedness (as defined in the Indenture), to the
same extent and in the same manner that the Debentures are subordinated to
Senior Indebtedness pursuant to the Indenture, (ii) pari passu with the
Debentures, the Other Debentures, the Series B Capital Securities Guarantee, the
Common Securities Guarantee and any Other Guarantee and any Other Common
Securities Guarantee, and (iii) senior to the Guarantor's capital stock.


                                  ARTICLE VII
                                  TERMINATION

SECTION 7.1   Termination

              This Series A Capital Securities Guarantee shall terminate and be
of no further force and effect (i) upon full payment of the Redemption Price (as
defined in the Declaration) of all Series A Capital Securities, (ii) upon
liquidation of the Issuer, the full payment of the amounts payable in accordance
with the Declaration or the distribution of the Debentures to the Holders and
the holders of the Common Securities or (iii) upon exchange of all the Series A
Capital Securities for the Series B Capital Securities in the Exchange Offer and
the execution and delivery of the Series B Capital Securities Guarantee.
Notwithstanding the foregoing, this Series A Capital Securities Guarantee will
continue to be effective or will be reinstated, as the case may be, if at any
time any Holder must restore payment of any sums paid under the Series A Capital
Securities or under this Series A Capital Securities Guarantee.


                                  ARTICLE VII
                                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 or claim incurred by reason 

                                       18
<PAGE>
 
of any act or omission performed or omitted by such Indemnified Person in good
faith in accordance with this Series A Capital 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 Series A Capital
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 Distributions 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 and all loss, liability,
damage, claim 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 (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 provisions in this
Section 8.2 shall survive the termination of this Series A Capital Securities
Guarantee or the resignation or removal of the Capital Securities Guarantee
Trustee.


                                  ARTICLE IX
                                 MISCELLANEOUS

SECTION 9.1   Successors and Assigns

              All guarantees and agreements contained in this Series  A Capital
Securities Guarantee shall bind the successors, assigns, receivers, trustees and
representatives of the Guarantor and shall inure to the benefit of the Holders
then outstanding.

SECTION 9.2   Amendments

              Except with respect to any changes that do not materially
adversely affect the rights of Holders (in which case no consent of Holders will
be required), this Series A Capital Securities Guarantee may only be amended
with the prior approval of the Holders of a Majority in liquidation amount of
the Series A Capital Securities (including the stated amount that would be paid
on 

                                       19
<PAGE>
 
redemption, liquidation or otherwise, plus accrued and unpaid Distributions to
the date upon which the voting percentages are determined). The provisions of
the Declaration with respect to consents to amendments thereof (whether at a
meeting or otherwise) shall apply to the giving of such approval.

SECTION 9.3   Notices

              All notices provided for in this Series A Capital Securities
Guarantee shall be in writing, duly signed by the party giving such notice, and
shall be delivered, telecopied or mailed by first class mail, as follows:

              (a)   If given to the Issuer, in care of the Administrative
Trustee at the mailing address set forth below (or such other address as the
Issuer may give notice of to the Holders and the Capital Securities Guarantee
Trustee):

                    NGC Corporation Capital Trust I   
                    c/o NGC Corporation               
                    1000 Louisiana, Suite 5800        
                    Houston, TX  77002                
                    Attention:  John U. Clarke        
                                Administrative Trustee
                    Telecopy: (713) 767-8322           

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

                    The First National Bank of Chicago            
                    One First National Plaza, Suite 0126          
                    Chicago, Illinois  60670-0126                 
                    Attention:  Corporate Trust Services Division 
                    Telecopy: (312) 407-1708                       

              (c)   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 and the Capital Securities Guarantee Trustee):

                    NGC Corporation                    
                    1000 Louisiana, Suite 5800         
                    Houston, TX  77002                 
                    Attention:  Kenneth E. Randolph 
                    Telecopy: (713) 507-6808

                                       20
<PAGE>
 
              (d)   If given to any Holder, 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   Exchange Offer

              In the event an Exchange Offer Registration Statement (as defined
in the Registration Rights Agreement) becomes effective and the Issuer issues
any Series B Capital Securities in the Exchange Offer, the Guarantor will enter
into a new capital securities guarantee agreement, in substantially the same
form as this Series A Capital Securities Guarantee, with respect to the Series B
Capital Securities.

SECTION 9.5   Benefit

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

SECTION 9.6   GOVERNING LAW

              THIS SERIES A CAPITAL SECURITIES GUARANTEE SHALL BE GOVERNED BY,
AND CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW
YORK.

SECTION 9.7   Limited Liability.

              The Holders of the Series A Capital Securities, in their
capacities as such, shall not be personally liable for any liabilities or
obligations of the Guarantor arising out of this Series A Capital Securities
Guarantee, and the parties hereby agree that the Holders of the Series A Capital
Securities, in their capacities as such, shall be entitled to the same
limitation of personal liability extended to the stockholders of private
corporations for profit organized under the General Corporation Law of the State
of Delaware.

              THIS SERIES A CAPITAL SECURITIES GUARANTEE is executed as of the
day and year first above written.

                                       21
<PAGE>
 
                                            NGC CORPORATION, as Guarantor


                                            By: ________________________________
                                            Name:
                                            Title:
 
                                            THE FIRST NATIONAL BANK OF CHICAGO,
                                            as Capital Securities Guarantee 
                                            Trustee



                                            By: ________________________________
                                            Name:
                                            Title:

                                       22