<Page>

                                                               Exhibit 10.11(a)

                              MILLENNIUM CHEMICALS

                             SUPPLEMENTAL EXECUTIVE

                                 RETIREMENT PLAN
                           (Effective January 1, 1999)





<Page>

Millennium Chemicals
Supplemental Executive Retirement Plan
(Effective January 1, 1999)

Contents

--------------------------------------------------------------------------------
Section                                                                     Page

                               ARTICLE I. THE PLAN

1.1  ESTABLISHMENT OF THE PLAN.................................................1
1.2  PURPOSE...................................................................1

                             ARTICLE II. DEFINITIONS

2.1  "AVERAGE FINAL COMPENSATION"..............................................1
2.2  "BOARD OF DIRECTORS"......................................................2
2.3  "CAUSE"...................................................................2
2.4  "COMMITTEE"...............................................................2
2.5  "COMPANY".................................................................2
2.6  "COMPENSATION"............................................................2
2.7  "DEFERRED RETIREMENT DATE"................................................2
2.8  "Good Reason".............................................................2
2.9  "Normal Retirement Date"..................................................2
2.10 "PARTICIPANT".............................................................2
2.11 "PENSION OFFSET"..........................................................3
2.12 "PLAN"....................................................................3
2.13 "PRIOR PLAN"..............................................................3
2.14 "PRIOR PLAN SERP".........................................................3
2.15 "QUALIFIED PLAN"..........................................................3

                            ARTICLE III. PARTCIPATION

3.1  PARTICIPATION.............................................................3
3.2  CONTINUATION OF PARTICIPATION.............................................3

                           ARTICLE IV. RETIREMENT DATE

4.1  NORMAL RETIREMENT DATE....................................................4
4.2  DEFERRED RETIREMENT DATE..................................................4
4.3  TERMINATED VESTED BENEFITS................................................4


                                       i





<Page>

Millennium Chemicals
Supplemental Executive Retirement Plan
(Effective January 1, 1999)

Contents

--------------------------------------------------------------------------------
Section                                                                     Page

                         ARTICLE V. RETIREMENT BENEFITS

5.1  NORMAL RETIREMENT BENEFIT.................................................4
5.2  DEFERRED RETIREMENT BENEFIT...............................................5
5.3  EARLY TERMINATION BENEFIT.................................................5
5.4  DISABILITY RETIREMENT BENEFIT.............................................6
5.5  ADJUSTED CREDITED SERVICE.................................................6

                        ARTICLE VI. DEATH BENEFITS

6.1  PRIOR TO RETIREMENT.......................................................6
6.2  AFTER RETIREMENT..........................................................7

               ARTICLE VII. TIME AND FORM OF BENEFIT PAYMENT

7.1  NORMAL FORM OF BENEFIT....................................................7
7.2  OPTIONAL FORMS OF BENEFIT.................................................7

                    ARTICLE VIII. PROVISION OF BENEFITS

8.1  PARTICIPANT CONTRIBUTIONS.................................................7
8.2  FUNDING...................................................................7

                  ARTICLE IX. ADMINISTRATION OF THE PLAN

9.1  POWERS AND DUTIES OF THE COMMITTEE........................................7

                         ARTICLE X. MISCELLANEOUS

10.1  PROHIBITION AGAINST ENCUMBRANCE..........................................8
10.2  RIGHT OF PARTICIPANT.....................................................8
10.3  CHANGE IN CONTROL........................................................8
10.4  SUCCESSORS..............................................................10

             ARTICLE XI. AMENDMENT OR TERMINATION OF THE PLAN

11.1  AMENDMENT...............................................................10


                                       ii





<Page>

11.2  TERMINATION.............................................................11


                                      iii





<Page>

Article I. The Plan

1.1 Establishment of the Plan

Effective January 1, 1999 Millennium Chemicals Inc. (the "Company") established
an unfunded supplemental retirement plan for select employees (the
"Participants") of Millennium America Holdings Inc., Millennium Inorganic
Chemicals Inc., Millennium Petrochemicals Inc., Millennium Specialty Chemicals
Inc. and such of the subsidiaries of the Company as the Committee may designate
at any time hereafter as participating employers hereunder (collectively the
"Employers" and individually the "Employer") which plan shall hereafter be known
as the "Millennium Chemicals Supplemental Executive Retirement Plan."

1.2 Purpose

The purpose of the Plan is to provide Participants with a minimum level of
retirement income from their Employer, in addition to other sources of capital
accumulation. The plan is intended to be a non-qualified, deferred compensation
plan for a "select group of management or highly compensated employees" as that
term is used in the Employee Retirement Income Security Act of 1974, as amended.

This Plan incorporates and replaces any agreement between a Participant and the
Company or any subsidiary of the Company with regard to non-qualified
supplemental retirement benefits that was in existence prior to January 1, 1999.

Article II. Definitions

Except as otherwise defined herein, each capitalized term shall have the meaning
set forth in the Qualified Plan.

2.1 "Average Final Compensation" shall mean the highest average annual
Compensation for the highest five calendar years in the last ten years of
Credited Service affording the highest such average, or during all years of
Credited Service if less than five. If a Participant has received Compensation
in less than five (5) calendar years immediately preceding the Participant's
Separation from Service Date, the average of a Participant's compensation for
calendar years and months preceding the Participant's Separation from Service
Date shall be utilized; provided that a participant's average monthly
Compensation shall be annualized by multiplying such average monthly
Compensation by twelve (12). Any administrative procedure adopted


                                       1





<Page>

relative to the calculation of Average Final Compensation for the Qualified Plan
shall apply to this Plan.

2.2 "Board of Directors" shall mean the Board of Directors of the Company except
as otherwise specifically stated.

2.3 "Cause" shall have the meaning determined from time to time by the
Committee, provided, however, that if the Participant has a written agreement
with the Company or any of its subsidiaries containing a definition of
termination for "Cause", then "Cause" shall have the meaning set forth in such
agreement.

2.4 "Committee" shall mean the Compensation Committee of the Board of Directors
of the Company.

2.5 "Company" shall mean Millennium Chemicals Inc. and its successors.

2.6 "Compensation" shall have the same meaning as in the Qualified Plan, except
that Compensation shall be determined without regard to the dollar limitations
on the amount of pay taken into account, which limitations are set forth in
section 2.1(k)(3) of the Qualified Plan. Compensation shall also be deemed to
include deferred salary and bonus which would have been included in the
definition of Compensation under the Qualified Plan if the salary or bonus had
not been deferred at the election of the Participant, but Compensation shall not
include the subsequent payment of any such deferred salary or bonus which has
been included in Compensation for a previous year.

2.7 "Deferred Retirement Date" shall mean the first day of the month coincident
with or immediately following the date a Participant retires after his or her
Normal Retirement Date pursuant to the provisions of section 4.2 (Deferred
Retirement Date).

2.8 "Good Reason" shall have the meaning determined from time to time by the
Committee, provided, however, that if the Participant has a written agreement
with the Company or any of its subsidiaries containing a definition of
termination for "Good Reason", then "Good Reason" shall have the meaning set
forth in such agreement.

2.9 "Normal Retirement Date" shall mean the first day of the month immediately
following the later of :

          (a)  the Participant's sixty-fifth (65th) birthday

                                       or

          (b)  the fifth (5th) anniversary of the date the Participant becomes a
               Member of the Qualified Plan.

2.10 "Participant" shall mean a person who has become a participant in this Plan
pursuant to section 3.1. Participants shall be limited to a "select group of
management


                                       2





<Page>

or highly compensated employees" as that term is used in the Employee Retirement
Income Security Act of 1974, as amended.

2.11 "Pension Offset" shall mean the benefit payable as a lump sum under the
Qualified Plan as of the determination date (reduced where applicable to reflect
payment of the benefit prior to Normal Retirement Date). The benefit payable
under the Qualified Plan shall not be less than the actuarial equivalent on the
determination date of a Participant's accrued benefit as at December 31, 1998
under a Prior Plan. Pension Offset shall also include any other pension
benefits, calculated in the same manner and form as stated above, required to be
offset under the Qualified Plan.

2.12 "Plan" shall mean the Millennium Chemicals Supplemental Executive
Retirement Plan.

2.13 "Prior Plan" shall mean any of the following plans:

The Millennium Chemicals Inc. Pension Plan;
The Millennium Inorganic Chemicals Inc. Salaried Employees Retirement Plan;
Pension Plan For Eligible Salaried and Non-Represented Employees of Millennium
Petrochemicals; and,
Millennium Specialty Chemicals Inc. Salaried Employees Retirement Plan

2.14 "Prior Plan SERP" shall mean a supplemental executive retirement plan
applicable to a Prior Plan.

2.15 "Qualified Plan" shall mean the Millennium Chemicals Consolidated
Retirement Plan or any successor plan.

Article III. Participation

3.1 Participation

Participants will be limited to individuals who are listed in Schedule A hereto.

3.2 Continuation of Participation

(a)  A person who has become a Participant in accordance with section 3.1 shall,
     except as provided in subsection (b) below, continue as a Participant as
     long as he or she is entitled to benefits under the Plan.


                                       3





<Page>

(b)  Subject to the provisions of section 10.3 (Change in Control) or as
     provided otherwise in a written agreement between a Participant and the
     Company or any of its subsidiaries, the Committee may, in its sole
     discretion, remove a Participant from active participation in the Plan. In
     such event, any benefits accrued under this Plan will be vested and payable
     at the Participant's Retirement Date in accordance with Article IV. Subject
     to the provisions of Section 10.3 and notwithstanding anything herein to
     the contrary other than Section 10.3, if a Participant's employment is
     terminated for Cause or if the Participant is convicted of a felony, all
     rights under this Plan shall be forfeited.

Article IV. Retirement Date

4.1 Normal Retirement Date

A Participant who retires on his or her Normal Retirement Date shall be entitled
to a benefit as determined in accordance with section 5.1 (Normal Retirement
Benefits).

4.2 Deferred Retirement Date

A Participant whose employment with the Employer continues beyond his or her
Normal Retirement Date and whose entitlement to benefits under the Plan has not
been forfeited in accordance with subsection (b) of section 3.2 (Continuation of
Participation), shall retire on a Deferred Retirement Date and shall be entitled
to a benefit in accordance with section 5.2 (Deferred Retirement Benefit).

4.3 Terminated Vested Benefits

A Participant who is eligible to receive a Terminated Vested Benefit under the
Qualified Plan if the Participant terminates his or her employment prior to
Normal Retirement Age shall be entitled to receive a benefit under Section 5.3
(Early Termination Benefit).

Article V. Retirement Benefits

5.1 Normal Retirement Benefit

The Normal Retirement Benefit payable hereunder shall be an amount expressed as
a Single Life Annuity commencing on Participant's Normal Retirement Date. Such
Annuity shall be the Actuarial Equivalent of a lump sum calculated as follows:


                                       4





<Page>

(a)  Participant's Average Final Compensation; multiplied by

(b)  The sum of his or her Annual Credits under the Qualified Plan; minus

(c)  the Pension Offset;

For a Participant who was under age 55 on January 1, 1999, the Normal Retirement
Benefit payable hereunder shall not be less than the Participant's accrued
benefit at December 31, 1998 under any Prior Plan SERP.

For a Participant who was age 55 on January 1, 1999, the Normal Retirement
Benefit payable hereunder shall not be less than the Participant's accrued
retirement benefit as of the determination date applying the formula contained
in the Prior Plan SERP.

A Participant's Normal Retirement Benefit shall commence on his or her Normal
Retirement Date provided his or her Qualified Plan Benefit commences on such
date.

5.2 Deferred Retirement Benefit

If a Participant remains in employment after his or her Normal Retirement Date
and is entitled to a benefit in accordance with section 4.2 (Deferred Retirement
Date), benefit payments shall be postponed until the Participant's actual
retirement on the Deferred Retirement Date. At such Deferred Retirement Date,
and provided the Qualified Plan Benefit then commences, the Participant shall be
entitled to the benefit determined under section 5.1 based on Credited Service
to the Deferred Retirement Date.

5.3 Early Termination Benefit

A Participant whose employment terminates other than for Cause prior to his or
her Normal Retirement Date shall be entitled to receive a benefit, commencing as
of the first day of the month following his or her termination of employment,
determined under the formula set forth in section 5.1 as of the date of such
termination.

In lieu of such benefit commencing on the first day of the month following his
or her termination of employment, the Participant may elect to receive such
benefit as a Terminated Vested Benefit on the first day of any month following
his or her termination of employment but not later than his or her Normal
Retirement Date, provided he or she elects to commence his or her benefit
payable under the Qualified Plan on the same date. In such case, the
Participant's benefit shall be adjusted to the Actuarial Equivalent of the
Participant's Terminated Vested Benefit determined as of the first day of the
month following his or her termination of employment.


                                       5





<Page>

5.4 Disability Retirement Benefit

Each Participant in active employment with the Company or an Affiliate who has
completed at least 5 years of Vesting service and whose employment terminates
because he or she incurs a Disability before his or her Normal Retirement Age,
shall be eligible to receive a Disability Retirement Benefit under the Plan. A
Participant's right to his or her Disability Retirement Benefit shall be 100
percent vested and nonforefeitable. A participant's Disability Retirement
Benefit shall be the Actuarial Equivalent (as defined in the Qualified Plan) of
the Participant's Normal Retirement Benefit determined under Section 5.1. Such
Disability Retirement Benefit shall be calculated as of the first day of any
month on which he or she elects to receive benefits following his or her
Separation from Service on account of Disability. A Participant cannot defer
commencement of his or her Disability Retirement Benefit to a date beyond his or
her Normal Retirement Date.

5.5 Adjusted Credited Service

The Committee may, in its sole discretion, determine an adjusted Credited
Service for the Participant. The adjusted Credited Service may be 1, 2, 3, 4, or
5 years more than the actual Credited Service (subject to the 35 years maximum
for Credited Service). Pursuant to this section 5.5 the Committee may in its
sole discretion cause a Participant to be become eligible for early retirement
under a Prior Plan.

In determining the amount of pension in accordance with this Article V, a
Participant's adjusted Credited Service shall be used as if it was his or her
actual Credited Service. However, under no circumstances shall benefits commence
under this Article V prior to commencement of benefits under the Qualified Plan.

Article VI. Death Benefits

6.1 Prior to Retirement

Upon the death prior to retirement of a Participant who has completed at least 5
years of Vesting Service, his or her surviving spouse, if any, shall be entitled
to a benefit hereunder if such surviving spouse is entitled to a benefit under
the Qualified Plan or one of the Prior Plans. The amount and form ( including
commencement date) of the benefit payable to the surviving spouse shall be
calculated in the same manner as the Spouse's Pension provided for in the
Qualified Plan, however, on the basis of the formula set forth in section 5.1
herein as of the date of Participant's death.


                                       6





<Page>

6.2 After Retirement

There is no benefit payable under the Plan in the Event of the Participant's
death after his or her retirement benefit has commenced unless an option is in
effect in accordance with section 7.2 (Optional Forms of Benefits).

Article VII. Time and Form of Benefit Payment

7.1 Normal Form of Benefit

Except as otherwise provided in section 7.2, the retirement benefit shall be
payable as a monthly annuity as of the first day of each calendar month for the
life of the Participant with benefits ceasing upon the Participant's death.

7.2 Optional Forms of Benefit

The benefit under this Plan may be paid in any form permissible under the
Qualified Plan regardless of how the benefit under the Qualified Plan is paid.
If such form of payment is other than a Single Life Annuity, the amount of
pension otherwise payable under this Plan shall be adjusted in the same manner
that benefits are to be adjusted under the Qualified Plan.

Article VIII. Provision of Benefits

8.1 Participant Contributions

Participants shall make no contributions under the Plan.

8.2 Funding

Benefit payments from the Plan will be made from the general assets of the
Company in accordance with such arrangements as the Company may deem necessary
and proper to fulfill its agreement hereunder.

Article IX. Administration of the Plan

9.1 Powers and Duties of the Committee

The Committee, in addition to all the powers and duties specified in the various
provisions of the Plan, shall have the exclusive right to interpret the Plan and
to decide any matter arising in connection with the administration of the Plan.


                                       7





<Page>

Article X. Miscellaneous

10.1 Prohibition Against Encumbrance

Except in the case of a court order which meets the requirements of a "qualified
domestic relations order" as defined in section 414(p) of the Internal Revenue
Code of 1986, no benefit under the Plan shall be alienated, assigned, disposed
of, or in any manner encumbered while in the possession and control of the
Company or the Employer. If the interest of any Participant or Participant's
beneficiary would, but for this section 10.1, cease in whole or in part to be
enjoyed by such individual, the Committee, in its sole discretion, may direct
that the funds constituting such interest be withheld or it may expend from such
funds for the direct maintenance and support of the Participant, such
Participant's spouse, children, or their dependents as, in the Committee's sole
discretion, it deems fit and proper.

10.2 Right of Participant

Neither the adoption of the Plan nor its operation shall in any way affect the
right and power of the Company or the Employer to dismiss or otherwise terminate
the employment, or change the terms of employment, or amount of compensation, of
any Participant at any time for any reason.

10.3 Change in Control

The Plan will terminate effective on the close of business 30 days following a
Change in Control, as hereinafter defined. Upon such Change in Control, section
11.1 shall become inoperative and, in addition, any modification or amendment
adopted within six months prior to the Change in Control shall be void with
respect to any specific Participant affected thereby unless approved by such
Participant. Upon such termination, each Participant (excluding Participants who
continue in active employment with the Company or an Employer, but including
each Participant whose employment was terminated within six months prior to such
Change in Control by the Employer without Cause, or due to disability, or
voluntarily for Good Reason as determined by the Committee and each Participant
that has experienced a Change of Control pursuant to a separate written "Change
in Control" agreement with the Company or the Employer) will be deemed retired
pursuant to Section 5.1 based on Benefit Service through the termination date. A
lump sum amount shall be calculated that is equal in value to each Participant's
Early Retirement Benefit payable under the Plan as of the date of the Change in
Control. Change in Control benefits payable to Participants who have not
attained 55 years of age shall be equal to the present value of such
Participant's Early Retirement Benefit payable at age 55. The calculation of
lump sum amounts hereunder shall be made using the prime rate of interest at
Citigroup, Inc. as of the date of the Change in Control or the rate applicable
to the Participant specified in the Qualified Plan, whichever is less, as the
discount rate to determine the present value of the early Retirement Benefit.
Benefits hereunder shall be paid as soon


                                       8





<Page>

as practicable following the date of the Change in Control, but in no event
later than 90 days thereafter whether or not the Participant has elected to
commence receiving benefits under the Qualified Plan. All other actuarial
assumptions and calculation methods to be used shall be those used in the
Qualified Plan document prior to such Change in Control.

For the purposes of this Plan, the term "Change in Control" shall mean (i) any
"person" as such term is used in Sections 13(d) and 14(d) of the Securities
Exchange Act of 1934 ("Act") (other than the Company, any trustee or other
fiduciary holding securities under any employee benefit plan of the Company or
any company owned, directly or indirectly, by the stockholders of the Company in
substantially the same proportions as their ownership of Common Stock of the
Company), becoming the "beneficial owner" (as defined in Rule 13d-3 under the
Act), directly or indirectly, of securities of the Company representing
twenty-five percent (25%) or more of the combined voting power of the Company's
then outstanding securities; (ii) during any period of two consecutive years
(not including any period prior to October 1, 1996), individuals who at the
beginning of such period constitute the Board of Directors of the Company, and
any new director (other than a director designated by a person who has entered
into an agreement with the Company to effect a transaction described in clause
(i), (iii), or (iv) of this paragraph or a director whose initial assumption of
office occurs as a result of either an actual or threatened election contest (as
such terms are used in Rule 14a-11 of Regulation 14A promulgated under the Act)
or other actual or threatened solicitation of proxies or consents by or on
behalf of a person other than the Board of Directors of the Company) whose
election by the Board of Directors or nomination for election by the Company's
stockholders was approved by a vote of at least two-thirds of the directors then
still in office who either were directors at the beginning of the two-year
period or whose election or nomination for election was previously so approved,
cease for any reason to constitute at least a majority of the Board of
Directors; (iii) the merger or consolidation of the Company with any other
corporation, other than a merger or consolidation which would result in the
voting securities of the Company outstanding immediately prior thereto
continuing to represent (either by remaining outstanding or by being converted
into voting securities of the surviving entity) more than fifty percent (50%) of
the combined voting power of the voting securities of the Company or such
surviving entity outstanding immediately after such merger or consolidation;
provided, however, that a merger or consolidation effected to implement a
recapitalization of the Company (or similar transaction) in which no person
(other than those covered by the exceptions in (i) above) acquires more than
twenty-five percent (25%) of the combined voting power of the Company's then
outstanding securities shall not constitute a Change in Control or (iv) the
stockholders of the Company approve a plan of complete liquidation of the
Company or the closing of the sale or disposition by the Company of all or
substantially all of the Company's assets other than the sale or disposition of
all or substantially all of the assets of the Company to one or more
Subsidiaries (as defined below) of the Company or to a person or persons who
beneficially own, directly or indirectly, at least fifty percent (50%) or more
of the combined voting power of the outstanding voting securities of the Company
at the time of the sale or disposition; provided, however, (y)


                                        9




<Page>

the sale or disposition of all or any part of the Company's interests in
Equistar (and all subsequent sales and dispositions of any securities or assets
received as proceeds thereof, or as proceeds of proceeds) shall not be deemed to
constitute a Change in Control, and (z) if the Company sells or disposes of
all or any part of the Company's interests in Equistar indirectly (either
through the sale or other disposition of any entity that owns, directly or
indirectly, all or any part of the Company's interests in Equistar, or
otherwise), then the sale or disposition of the Company's interests in Equistar
(and all subsequent sales and dispositions of any securities or assets received
as proceeds thereof, or as proceeds of proceeds) shall be ignored and
disregarded in determining whether any such Change in Control has occurred. By
way of illustration, if an indirect subsidiary of the Company that owns the
Company's interests in Equistar together with certain other assets is sold,
then, in determining whether a Change in Control has occurred, all relevant
determinations shall be made pursuant to the assumption that (y) such subsidiary
owns only such other assets, and (z) neither such subsidiary nor the Company
owns, either directly or indirectly, the interests in Equistar. "Subsidiary"
shall have the meaning set forth in Section 424 of the Code and the term shall
also include any partnership, limited liability company or other business entity
if the Company owns directly or indirectly, securities or other ownership
interests representing at least fifty percent (50%) of the ordinary voting power
or equity or capital interests of such entity. Only one (1) Change in Control
may take place under this Plan.

10.4 Successors.

In addition to any obligations imposed by law upon any successor to the Company
the Company will require any successor (whether direct or indirect, by purchase,
merger, consolidation or otherwise) to all or substantially all of the business
and/or assets of the Company to expressly assume and agree in writing to perform
the obligations of the Company under the Plan in the same manner and to the same
extent that the Company would be required to perform it if no such succession
had taken place and this Plan shall inure to the benefit of such successor. Any
such assignment shall not relieve the Company from liability hereunder for
periods prior to such assignment. Reference to the Company herein shall also
include any successor to the Company.

Article XI. Amendment or Termination of the Plan

11.1 Amendment

The Board of Directors reserves the right at any time and from time to time, to
modify or amend, in whole or in part, any or all of the provisions of the Plan
but no such


                                       10





<Page>

amendment shall adversely affect any Participant's or Participant's
beneficiary's rights to benefits accrued under the Plan prior the effective date
of the amendment.

11.2 Termination

The Board of Directors shall have the right to terminate the Plan at any time
provided that such action shall not adversely affect any Participant's or
Participant's beneficiary's rights to benefits accrued under the Plan prior to
such action.

                                   Schedule A

           Millennium Chemicals Supplemental Executive Retirement Plan

                        Employees Eligible to Participate


                                       11