This paper is concerned with the application of Web services to distributed, cross-organizational business processes. In this scenario, it is crucial to answer the following questions: Do two Web services fit together in a way such that the composed system is deadlock-free? – the question of compatibility. Can one Web service be replaced by another while the remaining components stay untouched? – the question of equivalence. Can we reason about the soundness of one given Web service without considering the actual environment it will by used in? This paper defines the notion of usability – an intuitive and locally provable soundness criterion for a given Web services. Based on this notion, this paper demonstrates how the other questions could be answered. The presented method is based on Petri nets, because this formalism is widely used for modeling and analyzing business processes. Due to the existing Petri net semantics for BPEL4WS – a language that is in the very act of bec...