Service orchestrations are a common means to compose individual services to either higher-level services or potentially complex composite applications. The Web Service Business Process Execution Language (WS-BPEL) is an example for a language that allows for defining automatically executable orchestrations of Web services. As of today, BPEL process are typically executed in a centralized manner; the process model is deployed on a single workflow management system which, during process instance execution, interprets the process definition and interacts with the orchestrated Web services on behalf of the user. In previous work, we have presented an approach which enables decentralized execution of BPEL processes based on a decentralized process model and supporting runtime infrastructure. In this paper we describe a method for automatic splitting of a process among the partners participating in its execution, referred to as process partitioning.