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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 y2elqeJHllyhXBqZbh7lHQ==

<SEC-DOCUMENT>0001140361-05-006874.txt : 20050830
<SEC-HEADER>0001140361-05-006874.hdr.sgml : 20050830
<ACCEPTANCE-DATETIME>20050830151356
ACCESSION NUMBER:		0001140361-05-006874
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050826
FILED AS OF DATE:		20050830
DATE AS OF CHANGE:		20050830

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			LINDEMANN GEORGE L
		CENTRAL INDEX KEY:			0000897631

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		051058624

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		5708202400

	MAIL ADDRESS:	
		STREET 1:		767 FIFTH AVENUE
		STREET 2:		50TH FLOOR
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10153

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:		417 LACKAWANNA AVENUE
		CITY:			SCRANTON
		STATE:			PA
		ZIP:			18503-2013
		BUSINESS PHONE:		(570) 614-5000

	MAIL ADDRESS:	
		STREET 1:		417 LACKAWANNA AVENUE
		CITY:			SCRANTON
		STATE:			PA
		ZIP:			18503-2013
</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>2005-08-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000897631</rptOwnerCik>
            <rptOwnerName>LINDEMANN GEORGE L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>767 FIFTH AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10153</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-08-26</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>146.1664</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>24.3</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>72402.90</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Supplemental Plan</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The Reporting Person also holds indirectly, 23,758 shares of common stock through the Issuer's 401(k) Plan and directly holds 3,337,814 shares of common stock and may be deemed to hold 3,289,220 shares owned by his wife, however, the Reporting Person disclaims beneficial ownership in such shares except to the extent of his pecuniary interest therein.</footnote>
        <footnote id="F2">This information concerns shares held under the Issuer?s Supplemental Deferred Compensation Plan.  This report also reflects shares purchased for the plan by the plan trustee, at the sole discretion of the trustee, at such prices as were available in the open market at the time of such purchases.</footnote>
        <footnote id="F3">All share amounts reported herein have been adjusted to reflect the 5% common stock dividend payable on September 1, 2005, to shareholders of record on August 22, 2005.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert M. Kerrigan, III for George L. Lindemann</signatureName>
        <signatureDate>2005-08-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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