The ItemsIds response group provides the ASIN for every item in the response to ItemLookup and SimilarityLookup operations. For ItemSearch, it also provides the total number of items and the total number of pages.
Sample Response (REST)
<?xml version="1.0" encoding="UTF-8"?>
<SimilarityLookupResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2004-08-01">
<OperationRequest>
<HTTPHeaders>
<Header Name="UserAgent" Value="Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)"/>
</HTTPHeaders>
<RequestId>018R4W0A19P3WPGEW54H</RequestId>
<Arguments>
<Argument Name="Service" Value="AWSECommerceService"/>
<Argument Name="AssociateTag" Value="[Your Associate ID Here]"/>
<Argument Name="SubscriptionId" Value="[Your Subscription ID Here]"/>
<Argument Name="ItemId" Value="0060006781"/>
<Argument Name="ResponseGroup" Value="ItemIds"/>
<Argument Name="Operation" Value="SimilarityLookup"/>
</Arguments>
</OperationRequest>
<Items>
<Request>
<IsValid>True</IsValid>
</Request>
<Item>
<ASIN>0060932902</ASIN>
</Item>
<Item>
<ASIN>0140264450</ASIN>
</Item>
<Item>
<ASIN>0060556579</ASIN>
</Item>
<Item>
<ASIN>0670031518</ASIN>
</Item>
<Item>
<ASIN>0618335404</ASIN>
</Item>
<Item>
<ASIN>0465044050</ASIN>
</Item>
<Item>
<ASIN>0670031860</ASIN>
</Item>
<Item>
<ASIN>0192860925</ASIN>
</Item>
<Item>
<ASIN>0393318486</ASIN>
</Item>
<Item>
<ASIN>0375415467</ASIN>
</Item>
</Items>
</SimilarityLookupResponse>
The Request that Generated the Response (REST)
http://webservices.amazon.com/onca/xml?Service=AWSECommerceService
&SubscriptionId=[Your Subscription ID Here]
&AssociateTag=[Your Associate ID Here]
&Operation=SimilarityLookup
&ItemId=0060006781
&ResponseGroup=ItemIds