The Open Grid Services Infrastructure (OGSI) defines a distributed system framework by integrating Grid and Web services technologies to facilitate resource sharing. In OGSI, Web services are supplemented with additional features in order to meet the requirements of Grid computing. However, the issue of Grid service composition is not well addressed in the OGSI framework. We apply BPEL4WS (Business Process Execution Language for Web Services) as a business workflow description language for the composition of Grid services. We provide an in depth analysis of BPEL4WS and OGSI in terms of their similarities and differences in areas such as life cycle management, Web service instantiation and instance group management. Based on our analysis we propose a high-level architecture to compliment OGSI with BPEL4WS for defining process workflow among Grid services. We describe a prototype system which shows how the proposed architecture can be used in modelling or orchestrating Grid services with BPEL4WS.