Service interoperability is a major obstacle in realizing the SOA vision. Interoperability is the capability of multiple, autonomous and heterogeneous systems to use each other’s services effectively. It is about the meaningful sharing of functionality and information that leads to the achievement of a common goal. In this paper we systematically explain what interoperability means and analyze possible interoperability problems. Further, we define requirements for service interoperability and present a method to assess whether a composite system meets the identified requirements