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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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<SEC-DOCUMENT>0001140361-08-012585.txt : 20080515
<SEC-HEADER>0001140361-08-012585.hdr.sgml : 20080515
<ACCEPTANCE-DATETIME>20080515142445
ACCESSION NUMBER:		0001140361-08-012585
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20080513
FILED AS OF DATE:		20080515
DATE AS OF CHANGE:		20080515

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:		5444 WESTHEIMER RD
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77056-5306
		BUSINESS PHONE:		(713) 989-2000

	MAIL ADDRESS:	
		STREET 1:		5444 WESTHEIMER RD
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77056-5306

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			JACOBI HERBERT H
		CENTRAL INDEX KEY:			0001197290

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		08836304

	MAIL ADDRESS:	
		STREET 1:		TRINKHAUS & BURKHARDT KOENSALLE 21/23
		STREET 2:		DUSSELFORF 1 D 40212
		CITY:			GERMANY
		STATE:			X9
		ZIP:			00000
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-05-13</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000203248</issuerCik>
        <issuerName>SOUTHERN UNION CO</issuerName>
        <issuerTradingSymbol>SUG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001197290</rptOwnerCik>
            <rptOwnerName>JACOBI HERBERT H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SOUTHERN UNION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>5444 WESTHEIMER ROAD</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056-5306</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-05-13</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>28250</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9830.738</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Directors' Deferred Compensation Plan</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The award of restricted shares was approved by the Board of Directors on May 13, 2008, upon a recommendation by the Compensation Committee and pursuant to the Southern Union Company Second Amended and Restated 2003 Stock and Incentive Plan.  The restrictions on the shares associated with this award expire on January 2, 2009.</footnote>
        <footnote id="F2">This information concerns shares held under the Issuer's Directors? Deferred Compensation Plan. This report reflects shares purchased for the plan by the plan trustee, at the sole discretion of the trustee, at such prices as were available on the open market at the time of such purchases.  Information reported herein is based on a plan statement for the period ended December 31, 2007.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert M. Kerrigan, III for Herbert H. Jacobi</signatureName>
        <signatureDate>2008-05-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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