A central requirement for achieving the vision of run-time discovery and dynamic composition of services is the provision of appropriate descriptions of the operation of a service, that is, how the service interacts with its environment, be it agents or other services. In this paper, we use experience gained through the development of real-life Grid applications to produce a set of requirements for such descriptions and then attempt to match those requirements against the offerings of existing work, such as OWL-S [1] and IRS-II [2]. Based on this analysis we identify which requirements are not addressed by current research and produce a model for describing the interaction protocol of a service in response. The main contributions of this model are the ability to describe the interactions of multiple parties with respect to a single service, distinguish between interactions initiated by the service itself and interactions that are initiated by clients or other cooperating services, and...