<?xml version="1.0" encoding="utf-8"?>
<InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.0.0.10</Version>
  <hasSegments>false</hasSegments>
  <ReportName>Note 18 - Acquisitions</ReportName>
  <ReportLongName>00917 - Disclosure - Note 18 - Acquisitions</ReportLongName>
  <DisplayLabelColumn>true</DisplayLabelColumn>
  <ShowElementNames>false</ShowElementNames>
  <RoundingOption />
  <HasEmbeddedReports>false</HasEmbeddedReports>
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="3 Months Ended" />
        <Label Id="2" Label="Mar. 31, 2010" />
      </Labels>
      <CurrencyCode>usd</CurrencyCode>
      <CurrencySymbol>$</CurrencySymbol>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <UnitID>Dollars</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema>
            <MeasureValue>USD</MeasureValue>
            <MeasureNamespace>iso4217</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>Shares</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>shares</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
      </Units>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>Notes to Financial Statements [Abstract]</Label>
      <Level>0</Level>
      <ElementName>pbra_NotesToFinancialStatementsAbstract</ElementName>
      <ElementPrefix>pbra</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ElementDataType>string</ElementDataType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDefenition>No definition available.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>Note 18 - Acquisitions</Label>
      <Level>1</Level>
      <ElementName>pbra_DomesticAndInternationalAcquisitionsTextBlock</ElementName>
      <ElementPrefix>pbra</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ElementDataType>string</ElementDataType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText>&lt;div style="PADDING-RIGHT: 0%; PADDING-LEFT: 0%"&gt;
  &lt;p style="text-align: justify;"&gt;&lt;b&gt;18. Acquisitions&lt;/b&gt;&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; &lt;b&gt;a) Sale option of the Pasadena refinery by Astra&lt;/b&gt;&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In a decision handed down on April 10, 2009, in the existing arbitration process between Petrobras America Inc - PAI and others and Astra Oil Trading NV - ASTRA and others, the exercise of the put option exercised by ASTRA with respect to PAI of the remaining 49.13% of the shares of ASTRA in Pasadena Refinery Systems Inc. (&amp;quot;PRSI&amp;quot;), was considered valid. The operating, management and financial responsibilities have already been transferred to PAI, based on preliminary decision of October 24, 2008.&lt;/p&gt;
&lt;/div&gt;
&lt;div style="PADDING-RIGHT: 0%; PADDING-LEFT: 0%"&gt;
  &lt;p style="text-align: justify;"&gt;According to the decision on April 10, the amount to be paid by PAI for the remaining shareholding interest in the refinery and in the trading company in Pasadena was fixed at US&amp;#36;466. The payment will be made in three installments, the first in the amount of US&amp;#36;296 (originally due on April 27, 2009, according to the decision) and the following two payments in the amount US&amp;#36;85 each, with due dates fixed by the arbitrators for September 2009 and September 2010. ASTRA presented a request for clarification to the arbitration panel on certain points of the decision.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; There are also judicial proceedings that are continuing in the progress aimed at defining, amongst other matters, aspects such as the partial confirmation/review of the arbitration report and requests, made by the parties, aimed at receiving reciprocal indemnities (in addition to those decided by the arbitrators) and the return by ASTRA of the books and documents of the companies&amp;#146; whose shares it sold and which it is withholding incorrectly.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In March 2009, a loss was recognized in the amount of US&amp;#36;147, corresponding to the difference between the fair value of the net assets and the value defined by the arbitration panel.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In April 2009, the Company recorded a charge of US&amp;#36;289 in as Additional Paid in Capital due to the acquisition of the remaining 49.13% of the shares of ASTRA in Pasadena Refinery Systems Inc. (&amp;quot;PRSI&amp;quot;), which relates to the difference between the fair value of the shares acquired and the noncontrolling interest carrying amount at the closing date.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; Until now the parties have not reached an agreement with respect to the finalization of various pending items existing between them, some of them, the object of double collection on the part of ASTRA, for signing the overall term of agreement that will put an end to the litigation and permit the payments that are the object of the arbitration decision.&lt;/p&gt;
&lt;/div&gt;
&lt;div style="PADDING-RIGHT: 0%; PADDING-LEFT: 0%"&gt;
  &lt;p style="text-align: justify;"&gt; On March 10, 2010, the Federal Court of Houston, Texas, USA, confirmed the arbitration award handed down on April 10, 2009, rejecting a request by PAI for extinguishment of the process without resolution of merit, through incompetence of the judge, and of partial annulment and modification of the arbitration award. It ratified, notwithstanding, the decision that PAI acquired 100% of the shareholding of Astra Oil Trading NV in PRSI. PAI requested reconsideration of the part of the decision that confirmed the competence of the Federal Court in question and other aspects of the decision.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; Judicial proceedings in which requests are made for reciprocal indemnifications made by the parties also continue in progress. Additionally, PRSI and the Trading Company are seeking recovery of certain accounting and tax books and records of these companies, incorrectly withheld by ASTRA and two legal firms.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; &lt;b&gt;b) Investment agreement between Petrobras, Petroquisa, Braskem, Odebrecht and Unipar&lt;/b&gt;&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; The investment agreement entered into on January 22, 2010, in accordance with a Material Fact disclosed to the market, established that the transaction for integration of the petrochemical interests will be achieved through the following steps: (i) formation of a holding company, BRK Investimentos Petroqu&amp;iacute;micos S.A. (&amp;#147;BRK&amp;#148;), which now holds all the common shares issued by Braskem previously held by Odebrecht, Petroquisa and Petrobras (Petroquisa and Petrobras, jointly, the &amp;#147;Petrobras System&amp;#148;); (ii) payments of funds into BRK, to be made in cash by Odebrecht and Petrobras; (iii) a capital increase from Braskem to be made in the form of a private subscription by its shareholders; (iv) acquisition by Braskem of the shares of Quattor Participa&amp;ccedil;&amp;otilde;es held by Unipar; (v) acquisition by Braskem of 100% of the shares of Unipar Comercial e Distribuidora S.A. (&amp;#147;Unipar Comercial&amp;#148;) and of 33.33% of the shares of Polibutenos S.A. Ind&amp;#250;strias Qu&amp;iacute;micas (&amp;#147;Polibutenos&amp;#148;); and (vi) incorporation by Braskem of the shares of Quattor held by the Petrobras System.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; On February 8, 2010, W.B.W., a subsidiary of Petroquisa, the holder of 31% of the voting capital of Braskem, was taken over by BRK. With this transaction, Odebrecht and the Petrobras System began the process for concentrating all their common shares issued by Braskem in BRK. As a result, BRK is now the holder of common shares issued by Braskem corresponding to 93.3% of its voting capital. The capital of BRK, in turn, was fully established through common shares held by Petroquisa and Odebrecht.&lt;/p&gt;
&lt;/div&gt;
&lt;div style="PADDING-RIGHT: 0%; PADDING-LEFT: 0%"&gt;
  &lt;p style="text-align: justify;"&gt; Also on February 8, 2010 a shareholders&amp;#146; agreement was entered into between the Petrobras System and Odebrecht which now regulates their relationship as shareholders of Braskem and BRK. The abovementioned shareholders&amp;#146; agreement reflects the commitments of the Petrobras System and Odebrecht to high levels of corporate governance and aggregation of value for all the shareholders of Braskem.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In the terms of the Shareholders&amp;#146; Agreement, all the issues within the competence of the General Shareholders&amp;#146; Meeting and the Board of Directors will be approved by consensus between Odebrecht and the Petrobras System. In the election of the Officers, including the Chief Executive Officer, and in the approval of the business plan, specific rules of the Shareholders&amp;#146; Agreement will be observed, which constitute an exception to the rule of consensus.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In compliance with what is established in the Investment Agreement, on March 30, 2010 Odebrecht contributed US&amp;#36;561 and, on April 5, 2010 Petrobras contributed US&amp;#36;1,404 to BRK. After the transfer of the abovementioned funds, Odebrecht and the Petrobras System now hold 53.79% and 46.21% of the total capital of BRK, respectively.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; Also, on January 22, 2010, Odebrecht, the Petrobras System and Braskem executed a joint-venture agreement, the purpose of which is to regulate their commercial and corporate relationship in the Petrochemical Complex of Suape (&amp;#147;Suape Complex&amp;#148;) and the Petrochemical Complex of the State of Rio de Janeiro (&amp;#147;COMPERJ&amp;#148;). The joint-venture agreement establishes that Braskem will gradually assume the companies that develop the businesses of the Suape Complex. With respect to the companies that develop the first and second petrochemical generations of COMPERJ, it was agreed that Braskem will assume these petrochemical businesses, observing the agreed-upon conditions. These transactions are in harmony with the interest of Odebrecht and the Petrobras System in integrating their petrochemical interests in Braskem.&lt;/p&gt;
&lt;/div&gt;
&lt;div style="PADDING-RIGHT: 0%; PADDING-LEFT: 0%"&gt;
  &lt;p style="text-align: justify;"&gt; The Company understands that the whole transaction is aligned with its strategic plan of operating in the petrochemical sector in a way that is integrated with its other businesses, adding value to its products and permitting more effective participation in Braskem.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; &lt;b&gt;c) Petrobras Biocombust&amp;iacute;vel acquires an interest in an ethanol refinery&lt;/b&gt;&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; In the first quarter of 2010, Petrobras Biocombust&amp;iacute;vel paid US&amp;#36;59 into the capital of Total Agroind&amp;#250;stria Canavieira S.A., in accordance with a commitment established in the Minutes of the Special General Shareholders&amp;#146; Meeting of December 22, 2009, to pay in the amount of US&amp;#36;84 not later than March 2011, when it will then hold 40.4% of the capital.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; This initiative, in line with strategic planning for 2009-2013, inserts the Company in the ethanol market. The partnership will make it viable to expand the refinery to a total capacity of 203 million liters per year, with surplus electric power of 38.5 MW for trading, generated through the use of sugar cane bagasse.&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; &lt;b&gt;d) Increase in the interest in the capital of Breitener Energ&amp;eacute;tica S.A.&lt;/b&gt;&lt;/p&gt;
  &lt;p style="text-align: justify;"&gt; Untill December 31, 2009, Petrobras held 30% of the capital of Breitener Energ&amp;eacute;tica S.A., a company established for the purpose of generating electric power, situated in the city of Manaus, in the state of Amazonas. On February 12, 2010, 35% of the interest in the capital was purchased for US&amp;#36;2 and Petrobras now holds shareholding control of the company. The evaluation of the fair value of the assets and liabilities has not been concluded and, therefore, preliminarily, a gain of US&amp;#36;11 was recognized in earnings for the period.&lt;/p&gt;
&lt;/div&gt;</NonNumbericText>
          <NonNumericTextHeader>18. Acquisitions
   a) Sale option of the Pasadena refinery by Astra
   In a decision handed down on April 10, 2009, in the existing arbitration process</NonNumericTextHeader>
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDefenition>No definition available.</ElementDefenition>
      <ElementReferences>No authoritative reference available.</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <ComparabilityReport>false</ComparabilityReport>
  <NumberOfCols>1</NumberOfCols>
  <NumberOfRows>2</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
