-----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,
 ESkFlUJI217nsYAN2Gz5kFoBNhA0FlGPgDrK8/LpkhIJWD3uMCDD0UAHI7RKJ2oZ
 LosmAT0MCl4p0J18LzqRVw==

<SEC-DOCUMENT>0001140361-05-001378.txt : 20050215
<SEC-HEADER>0001140361-05-001378.hdr.sgml : 20050215
<ACCEPTANCE-DATETIME>20050215174234
ACCESSION NUMBER:		0001140361-05-001378
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050211
FILED AS OF DATE:		20050215
DATE AS OF CHANGE:		20050215

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			McCarter Thomas N III
		CENTRAL INDEX KEY:			0001315247

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-06407
		FILM NUMBER:		05618453

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		570-820-2400

	MAIL ADDRESS:	
		STREET 1:		C/O SOUTHERN UNION COMPANY
		CITY:			WILKES-BARRE
		STATE:			PA
		ZIP:			18711

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
</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>2005-02-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001315247</rptOwnerCik>
            <rptOwnerName>McCarter Thomas N III</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>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Purchase Contract</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-02-11</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>200</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>50</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The purchase contract, together with a 5% undivided beneficial interest in $1,000 principal amount of a Southern Union senior note due February 16, 2008, comprise a Southern Union Corporate Unit.  The purchase contract obligates the Reporting Person to purchase from Southern Union, no later that February 16, 2008, for a price of $50 in cash, the following number of shares of Southern Union common stock, subject to anti-dilution adjustments:
If the average closing price of Southern Union common stock over the 20-day period ending on the third trading day prior to February 16, 2008 equals or exceeds $30.7625, 1.6254 shares; if the average closing price of Southern Union common stock over the same period is less than $30.7625 but greater than $24.61, a number of shares having a value, based on the average closing price, equal to $50: and if the average closing price of Southern Union common stock over the same period is less than or equal to $24.61, 2.0317 shares.</footnote>
        <footnote id="F2">The reporting person directly owns 1,000 shares of common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Dennis K. Morgan for Thomas N. McCarter, III</signatureName>
        <signatureDate>2005-02-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----