In the area of business processes, services needed and provided by organizations are more and more increasing, especially with the emergence of new technologies, such as workflow and web services supported by Service Oriented Architectures (SOA). The two technologies aim to provide flexibility, scalability and efficiency for business applications and to improve collaboration between business partners. This paper lies at a conceptual level, it proposes an approach to connect workflows of several partners using services. The approach is supported by a process meta-model which combines workflow concepts and SOA concepts, for modeling inter-organizational processes particularly built according to a subcontracting architecture. The advantage of using an approach based on services is to obtain process models flexible enough in order to allow easier adaptation in case of new business needs, because services are loosely coupled components. Our approach is illustrated by instancing concepts on a simple example of inter-organizational process.