<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-01-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001323640</rptOwnerCik>
            <rptOwnerName>Bond Robert O</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5150 WESTHEIMER ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056-5306</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP, Pipeline Operations</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-01-09</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1457</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>42.2625</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k) Savings Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to purchase)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>16.8255</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2014-02-06</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>16538</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16538</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to purchase)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>22.68</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-11-11</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>100000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>100000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.07</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2016-12-28</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>23753</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23753</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.48</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-12-17</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>60445</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>60445</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Cash Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7732</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7732</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F5"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-12-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>95336</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>95336</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Cash Restricted stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7470</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7470</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-12-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>39329</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>39329</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Cash Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F8"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>9681</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9681</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F9"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F9"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-12-13</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>32168</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>32168</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Cash Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F10"/>
                <footnoteId id="F11"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F10"/>
                <footnoteId id="F11"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F10"/>
                <footnoteId id="F11"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7724</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7724</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On July 19, 2011, the Company entered into a Second Amended and Restated Agreement and Plan of Merger with Energy Transfer Equity, L.P. (&quot;ETE&quot;) and Sigma Acquisition Corporation, a wholly-owned subsidiary of ETE, as amended by Amendment No. 1 to Agreement and Plan of Merger dated as of September 14, 2011 (the &quot;Merger Agreement&quot;).  Pursuant to the Merger Agreement, ETE exercised its right to request that the Company eliminate from the Southern Union Savings Plan (the &quot;Savings Plan&quot;) all shares of Company common stock. Accordingly, the trustee of the Savings Plan liquidated all shares of Company common stock held in the Savings Plan, including those held by the Reporting Person. The share price reflected  represents the average sale price realized by the trustee of the Savings Plan in open market trading on all shares held in the Savings Plan.</footnote>
        <footnote id="F2">The options are fully vested and exercisable.</footnote>
        <footnote id="F3">Stock appreciation rights will be settled in shares of common stock at an exercise price of $28.48 per share, which was equal to the closing price on Grant Date 2007.  The award vests in equal installments on the first, second and third anniversaries of Grant Date 2007.  As such, 40,296 stock appreciation rights are currently exercisable. The remaining 20,149 stock appreciation rights will vest on December 17, 2010.</footnote>
        <footnote id="F4">23,195 cash restricted stock units were awarded to the reporting person on December 15, 2008 (&quot;Grant Date 2008&quot;).  The cash restricted stock units permit the recipient to receive, on predetermined dates upon expiration of applicable restrictions, cash in an amount equal to a specified number of shares of the Company's common stock valued at the closing price of the Company's common stock on such dates.  Restrictions on each award expire in equal  installments on the first, second and third anniversaries of Grant Date 2008. To date, restrictions have expired on 7,731 of the cash restricted stock units. Restrictions on an additional 7,732 cash restricted stock units will expire on December 15, 2010.</footnote>
        <footnote id="F5">Stock appreciation rights will be settled in shares of common stock at an exercise price of $12.55 per share, which was equal to the closing price on Grant Date 2008.  The award vests in equal  installments on the first, second and third anniversaries of Grant Date 2008. As such, 31,778 stock appreciation rights are currently exercisable. An additional 31,779 stock appreciation rights will vest on December 15, 2010.</footnote>
        <footnote id="F6">11,204 cash restricted stock units were awarded to the Reporting Person on December 15, 2009 (&quot;Grant Date 2009&quot;). The cash restricted stock units permit the recipient to receive, on predetermined dates upon expiration of applicable restrictions, cash in an amount equal to a specified number of shares of the Company's common stock valued at the closing price of the Company's common stock on such dates.  Restrictions on the award will expire in equal  installments on the first, second and third anniversaries of Grant Date 2009. Restrictions on 3,734 cash restricted units expired on December 15, 2010.</footnote>
        <footnote id="F7">Stock appreciation rights will be settled in shares of common stock at an exercise price of $21.64 per share, which was equal to the closing price on Grant Date 2009.  The award vests in equal  installments on the first, second and third anniversaries of Grant Date 2009. 13,109 stock appreciation rights will vest on December 15, 2010.</footnote>
        <footnote id="F8">9,681 cash restricted stock units were awarded to the Reporting Person on December 13, 2010 (&quot;Grant Date 2010&quot;). The cash restricted stock units permit the recipient to receive, on predetermined dates upon expiration of applicable restrictions, cash in an amount equal to a specified number of shares of the Company's common stock valued at the closing price of the Company's common stock on such dates.  Restrictions on the award will expire in equal  installments on the first, second and third anniversaries of Grant Date 2010.</footnote>
        <footnote id="F9">Stock appreciation rights will be settled in shares of common stock at an exercise price of $24.80 per share, which was equal to the closing price on Grant Date 2010.  The award vests in equal  installments on the first, second and third anniversaries of Grant Date 2010.</footnote>
        <footnote id="F10">7,724 cash restricted stock units (&quot;RSUs&quot;) were awarded to the Reporting Person on November 6, 2011 (&quot;Grant Date 2011&quot;).  The RSUs were awarded in accordance with the Company's Third Amended and Restated 2003 Stock and Incentive Plan. The RSUs permit the Reporting Person to receive cash in an amount equal to a specified number of shares of the Company's common stock valued at the closing price of the Company's common stock on the dates such restrictions expire. Restrictions on such award will expire in equal amounts on the first, second and third anniversaries of Grant Date 2011, unless the restrictions are earlier terminated in accordance with the Plan or the applicable award agreement.</footnote>
        <footnote id="F11">On July 19, 2011, the Company entered into a Second Amended and Restated Agreement and Plan of Merger with Energy Transfer Equity, L.P. (&quot;ETE&quot;) and Sigma Acquisition Corporation, a wholly-owned subsidiary of ETE (as amended by Amendment No. 1 to Agreement and Plan of Merger dated as of September 14, 2011, the &quot;Merger Agreement&quot;).   The RSUs awarded to the Reporting Person will be treated in accordance with the terms and conditions of the Merger Agreement.  The Merger Agreement is available in its entirety as Annex A to the Company's definitive Proxy Statement filed with the SEC on October 27, 2011.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert M. Kerrigan, III for Robert O. Bond</signatureName>
        <signatureDate>2012-01-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
