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Proc-Type: 2001,MIC-CLEAR
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<SEC-DOCUMENT>0001140361-05-005253.txt : 20050715
<SEC-HEADER>0001140361-05-005253.hdr.sgml : 20050715
<ACCEPTANCE-DATETIME>20050715125209
ACCESSION NUMBER:		0001140361-05-005253
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050713
FILED AS OF DATE:		20050715
DATE AS OF CHANGE:		20050715

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SOUTHERN UNION CO
		CENTRAL INDEX KEY:			0000203248
		STANDARD INDUSTRIAL CLASSIFICATION:	NATURAL GAS DISTRIBUTION [4924]
		IRS NUMBER:				750571592
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		ONE PEI CENTER
		CITY:			WILKES-BARRE
		STATE:			PA
		ZIP:			18711
		BUSINESS PHONE:		(570) 820-2400

	MAIL ADDRESS:	
		STREET 1:		ONE PEI CENTER
		CITY:			WILKES-BARRE
		STATE:			PA
		ZIP:			18711

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ROBILLARD THOMAS C
		CENTRAL INDEX KEY:			0001184015

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		05956543

	BUSINESS ADDRESS:	
		STREET 1:		C/O FLEISCHMAN & WALSH LLP
		STREET 2:		1919 PENNSYLVANIA AVE., NW #600
		CITY:			WASHINGTON
		STATE:			DC
		ZIP:			20006
		BUSINESS PHONE:		5708202400

	MAIL ADDRESS:	
		STREET 1:		C/O SOUTHERN UNION CO
		STREET 2:		ONE PEI CENTER
		CITY:			WILKES BARRE
		STATE:			PA
		ZIP:			18711-0601
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

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    <periodOfReport>2005-07-13</periodOfReport>

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        <issuerName>SOUTHERN UNION CO</issuerName>
        <issuerTradingSymbol>SUG</issuerTradingSymbol>
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            <rptOwnerName>ROBILLARD THOMAS C</rptOwnerName>
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            <rptOwnerStreet1>C/O SOUTHERN UNION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>417 LACKAWANNA AVENUE</rptOwnerStreet2>
            <rptOwnerCity>SCRANTON</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>18503</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
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        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President of Division</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

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                </transactionShares>
                <transactionPricePerShare>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>7875</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>15.38</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2002-06-05</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2011-06-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7875</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7784</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The Reporting Person also owns indirectly, 2,698 shares of common stock through the Issuer?s 401(K) Plan and 35,016 shares of common stock through the Issuer's Supplemental Plan.</footnote>
        <footnote id="F2">The options vested in five equal annual installments begining on the first anniversary of the grant date.</footnote>
        <footnote id="F3">Of the remaining 7,874 options, 6,989 are currently exercisable and 795 are exercisable on July 1, 2006.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert M. Kerrigan, III for Thomas C. Robillard</signatureName>
        <signatureDate>2005-07-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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