The SOAP protocol underpins Web services as the standard mechanism for exchanging information in a distributed environment. The XML-based protocol offers advantages including extensibility, interoperability, and robustness. The merger of Web services and grid computing promotes SOAP into a standard protocol for the large-scale scientific applications that computational grids promise to support, further elevating the protocol’s importance and requiring highperformance implementations. Various SOAP implementations differ in their implementation language, invocation model and API, and supported performance optimizations. In this paper we compare and contrast the performance of widely used SOAP toolkits and draw conclusions about their current performance characteristics. We also provide insights into various design features that can lead to optimized SOAP implementations. The SOAP implementations included in our study are gSOAP 2.4, AxisC++ CVS May 28,