<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-02-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001018099</issuerCik>
        <issuerName>MILLENNIUM CHEMICALS INC</issuerName>
        <issuerTradingSymbol>MCH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001030536</rptOwnerCik>
            <rptOwnerName>LUSHEFSKI JOHN E</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MILLENNIUM CHEMICALS INC</rptOwnerStreet1>
            <rptOwnerStreet2>20 WIGHT AVENUE, SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>HUNT VALLEY</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>21030</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Exec. VP &amp; CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock $0.01 par value/share</value>
            </securityTitle>
            <transactionDate>
                <value>2004-02-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>64</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>12.65</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>19784</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents the value of the Reporting Person's Company Stock Fund Account in the Company's 401(k) plan as of February 2, 2004, expressed as share equivalents.  As of such date, approximately 96.6% of the Company Stock Fund was invested in Company Common Stock, and the remainder was invested in cash.  Please note that share equivalent accounting affects the total in column 5.

Please refer to attached Exhibit 99 for &quot;Amount of Securities Owned Following Required Transaction(s) in Table I and also for totals of Derivative Securities previously reported in Table II.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>John E. Lushefski</signatureName>
        <signatureDate>2004-02-04</signatureDate>
    </ownerSignature>
</ownershipDocument>

