This paper discusses the design for an efficient response cache mechanism appropriate for the Web services architecture. The important feature of Web services is its interoperability between heterogeneous platforms. This interoperability is based on widely accepted standards such as XML, SOAP, and WSDL. We describe a response cache mechanism for Web services client middleware without any extensions to these standards so that the client can participate transparently in the existing Web services community. We propose three optimization methods to improve the performance of our response cache. The first optimization is caching the post-parsing representation instead of the XML message itself. The second is caching application objects. For this optimization, we show some copying processes that are dependent on the type of cached objects. The third optimization is for read-only objects. These methods reduce the overhead of XML processing or object copying. We have implemented a prototype of a response cache on Apache-Axis, and evaluated these optimization methods through experiments for Google Web services. Finally, based on the experimental results, we discuss the optimal configuration of these methods based on data types.