<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>1090 - Disclosure - Rentals under Operating Leases</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2010 - 12/31/2010
USD ($)

USD ($) / shares

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&lt;tr&gt;
&lt;td&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&lt;b&gt;9.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Rentals under Operating Leases&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Future minimum rentals to be received under noncancelable tenant operating leases for each of the next five years and thereafter, excluding tenant reimbursements of operating expenses and percentage rent based on tenant sales volume as of December&amp;nbsp;31, 2010 are as follows: &lt;/font&gt;&lt;/p&gt;
&lt;div style="PADDING-RIGHT: 0pt; PADDING-LEFT: 0pt; PADDING-BOTTOM: 0pt; MARGIN-LEFT: 20%; WIDTH: 60%; PADDING-TOP: 0pt; POSITION: relative"&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&lt;!-- COMMAND=ADD_TABLEWIDTH,"100%" --&gt;&lt;/font&gt;&lt;/p&gt;
&lt;!-- User-specified TAGGED TABLE --&gt;
&lt;div align="center"&gt;
&lt;table cellspacing="0" cellpadding="0" width="100%" border="0"&gt;
&lt;tr style="HEIGHT: 0px"&gt;&lt;!-- TABLE COLUMN WIDTHS SET --&gt;
&lt;td style="FONT-FAMILY: times" align="left"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right" width="6"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="61"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;&lt;!-- TABLE COLUMN WIDTHS END --&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="#CCEEFF"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;2011&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;2,140,123&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="white"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;2012&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;1,944,020&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="#CCEEFF"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;2013&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;1,709,738&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="white"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;2014&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;1,501,966&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="#CCEEFF"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;2015&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;1,252,753&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="white"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;Thereafter&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;3,369,238&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="FONT-SIZE: 1.5pt; HEIGHT: 0px" valign="top"&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="BORDER-BOTTOM: #000000 1pt solid; FONT-FAMILY: times" valign="bottom" align="right" colspan="2"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="#CCEEFF"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&amp;nbsp;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;11,917,838&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="FONT-SIZE: 1.5pt; HEIGHT: 0px" valign="top"&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="BORDER-BOTTOM: #000000 2.25pt double; FONT-FAMILY: times" valign="bottom" align="right" colspan="2"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;
&lt;!-- end of user-specified TAGGED TABLE --&gt;&lt;/div&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Approximately 0.6% of future minimum rents to be received are attributable to leases with an affiliate of a limited partner in the Operating Partnership. &lt;/font&gt;&lt;/p&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;
</NonNumbericText><NonNumericTextHeader>9.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Rentals under Operating Leases
&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Future minimum rentals to be</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>Provide a general description of lessor's leasing arrangements for operating leases, including the basis on which contingent rental payments are determined, the existence and terms of renewal or purchase options and escalation clauses, restrictions imposed by lease arrangements, such as those concerning dividends, additional debt, and further leasing, rent holidays, rent concessions, or leasehold improvement incentives and unusual provisions or conditions.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name Statement of Financial Accounting Standard (FAS)
 -Number 13
 -Paragraph 23
 -Subparagraph c

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