Abstract The Web Service Business Process Execution Language (WSBPEL) is the de facto standard for describing workflow-like compositions of Web services, so-called Web service orchestrations. In this paper an ontology for executable BPEL processes is presented, which reflects both the natural language description and the syntax given in the specification. The ontology makes BPEL process models accessible at a semantic level and thus to intelligent queries and machine reasoning. Key words: BPM, BPEL, ontology, semantics, WSML