PEWS is a language for the implementation of web service interfaces. PEWS programs can be used for the description of both individual and composed web services. Individual web services can be built up from Java programs. Composed web services are built from simpler services. PEWS operators describe the allowed workflow of the web service, i.e.the order in which the operations of the web service will be executed. In this paper we analyze the expressiveness of PEWS programs. This is done by the systematic evaluation of the language. Our evaluation is based on a framework composed by workflow patterns. We also compare PEWS with other interface description languages. This comparison is based on the workflow behavior of the languages.