-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 BxcGkLNTHFFeWD54Bmlw7D0VzFkcooDTy1T/qCDPweMVfm1gLyIqNA3coiKnbRte
 2T1Mk/G3YFzXWgTmIS4zcA==

<SEC-DOCUMENT>0001140361-05-001110.txt : 20050209
<SEC-HEADER>0001140361-05-001110.hdr.sgml : 20050209
<ACCEPTANCE-DATETIME>20050209145742
ACCESSION NUMBER:		0001140361-05-001110
CONFORMED SUBMISSION TYPE:	5
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20041231
FILED AS OF DATE:		20050209
DATE AS OF CHANGE:		20050209

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:			DENIUS FRANK W
		CENTRAL INDEX KEY:			0001183998

	FILING VALUES:
		FORM TYPE:		5
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		05588311

	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>5
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 5
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>5</documentType>

    <periodOfReport>2004-12-31</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <form3HoldingsReported>0</form3HoldingsReported>

    <form4TransactionsReported>0</form4TransactionsReported>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001183998</rptOwnerCik>
            <rptOwnerName>DENIUS FRANK W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SOUTHERN UNION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>ONE PEI CENTER</rptOwnerStreet2>
            <rptOwnerCity>WILKES-BARRE</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>18711</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>1988-08-08</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1835.685</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>17820.250</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Director's Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This information concerns shares held under the Issuer?s Director?s Deferred Compensation Plan.  This report reflects shares purchased for the plan by the plan trustee, at such prices as were available in the open market at the time of such purchases.  Information reported herein is based on a plan statement for the period ended December 31, 2004.</footnote>
        <footnote id="F2">The Reporting Person may also be deemed to own directly 30,742 shares of common stock and indirectly 1,159 shares owned by his wife and 65,655 shares by the Effie and Wofford Cain Foundation. The  Reporting Person disclaims beneficial ownership of these shares and this report should not be deemed an admission that the Reporting Person is the beneficial owner of such shares for purposes of Section 16 or any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Dennis K. Morgan for Frank W. Denius</signatureName>
        <signatureDate>2005-02-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----