Service-oriented computing as a concept for providing interoperability and flexibility within heterogeneous environments has gained much attention within the last few years. Dynamically integrating external Web services into enterprise applications requires automatic contracting between service requestors and providers and automatic contract monitoring. This paper suggests a semi-automatic approach since in the current legal environment full automation is not feasible. We elaborate on the content of Web service contracts from a legal perspective and derive a set of legal requirements. Based on these requirements we propose an ontology-based representation of contract clauses as well as monitoring information. We can thus automatically evaluate whether a service execution meets the requirements expressed in a contract.