<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>0013 - Disclosure - 6. ACQUISITION OF TRANSTECH SYSTEMS, INC.</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>From2012-10-01to2013-06-30</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001074828</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2012-10-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="9 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column><Column FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>From2011-10-01to2012-09-30</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001074828</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2011-10-01T00:00:00</PeriodStartDate><PeriodEndDate>2012-09-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="12 Months Ended" /><Label Key="Calendar" Id="1" Label="Sep. 30, 2012" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>VSUL_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>VSUL_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Notes to Financial Statements</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>
</LabelSeparator><Level>2</Level><ElementName>VSUL_AcquisitionOfTranstechSystemsInc.TextBlock</ElementName><ElementPrefix>VSUL_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="From2012-10-01to2013-06-30" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;Our wholly owned subsidiary, TransTech Systems,is based in Aurora, Oregon, and was founded in 1994. TransTech provides value-added security and authentication solutions to corporate
and government security and law enforcement markets throughout the United States. &amp;#160;&amp;#160;With recorded revenues of $7.6 million
in 2012, TransTech has a respected national reputation for outstanding product knowledge, sales and service excellence.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company closed the acquisition of TransTech
on June 8, 2010. The Company acquired its 100% interest in TransTech by issuing a Promissory Note to James Gingo, the President
of TransTech, in the amount of $2,300,000, plus interest at the rate of three and one-half percent per annum from the date of the
Note. The Note was secured by a security interest in the stock and assets of TransTech, and was payable over a period of three
years. The final balance of $1,000,000 on the Note and accrued interest of $30,397 were paid to Mr. Gingo on June 12, 2013, to
complete the purchase price for the TransTech stock.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On June 8, 2010 in connection with the acquisition
of TransTech, the Company issued a total of 3,800,000 shares of restricted common stock of the Company to James Gingo, Jeff Kruse
and Steve Waddle, executives of TransTech, and Paul Bonderson, a TransTech investor. &amp;#160;The parties valued the shares in this
transaction at $76,000 or $0.02 per share, the closing bid price during negotiations.&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;This acquisition was entered into to accelerate
market entry and penetration through well-operated and positioned dealers of security and authentication systems, thus creating
a natural distribution channel for products featuring the Company&amp;#146;s proprietary ChromaID technology.&lt;/p&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell><Cell FlagID="0" ContextID="From2011-10-01to2012-09-30" UnitID=""><Id>2</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="margin: 0pt"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&lt;b&gt;6.&amp;#160;&lt;/b&gt;&amp;#9;&lt;b&gt;ACQUISITION OF TRANSTECH&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company closed the acquisition of
TransTech of Aurora, Oregon on June 8, 2010. On this date, the Company entered into a Stock Purchase, Security and Stock Pledge
Agreements which are included as Exhibits to the Form 10-Q filed with the SEC on August 12, 2010.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;TransTech, founded in 1994, is a distributor
of access control and authentication systems serving the security and law enforcement markets. &amp;#160;&amp;#160;With recorded revenues
of $7.6 million in 2012, TransTech has a respected national reputation for outstanding product knowledge, sales and service excellence.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;This acquisition is expected to accelerate
market entry and penetration through the acquisition of well-operated and positioned distributors of security and authentication
systems like TransTech, thus creating a natural distribution channel for products featuring the company&amp;#146;s proprietary Spectral
Pattern Matching technology.&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;The Company acquired its 100% interest
in TransTech by issuing a Promissory Note (&amp;#147;Note&amp;#148;) to James Gingo, the President of TransTech, in the amount of $2,300,000,
plus interest at the rate of three and one-half percent (3.5%) per annum from the date of the Note. The Note is secured by a security
interest in the stock and assets of TransTech, and is payable over a period of three (3) years.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify"&gt;On June 8, 2010, the Company issued a
total of 3,800,000 shares of restricted common stock of the Company to James Gingo, Jeff Kruse and Steve Waddle, executives of
TransTech, and Paul Bonderson, a TransTech investor. &amp;#160;The parties valued the shares in this transaction at $76,000 or $0.02
per share, the closing bid price during negotiations.&lt;/p&gt;



&lt;p style="margin: 0pt"&gt;&lt;/p&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>Acquisition of transtech systems inc.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>ACQUISITION OF TRANSTECH SYSTEMS, INC.</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>6. ACQUISITION OF TRANSTECH SYSTEMS, INC.</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://visualant.net/role/AcquisitionOfTranstechSystemsInc.</RoleURI><NumberOfCols>2</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
