Nowadays, applications are increasingly deployed as Web services in the globally distributed cloud computing environment. Multiple services are normally composed to fulfill complex functionalities. Business Process Execution Language for Web Services (WS-BPEL) is an XML-based service composition language that is used to define a complex business process by orchestrating multiple services. Compared with traditional applications, WS-BPEL programs pose many new challenges to the quality assurance, especially testing, of service compositions. A number of techniques have been proposed for testing WS-BPEL programs, but only a few studies have been conducted to systematically evaluate the effectiveness of these techniques. Mutation testing has been widely acknowledged as not only a testing method in its own right but also a popular technique for measuring the fault-detection effectiveness of other testing methods. Several previous studies have proposed a family of mutation operators for generating mutants by seeding various faults into WS-BPEL programs. In this study, we conduct a series of empirical studies to evaluate the applicability and effectiveness of various mutation operators for WS-BPEL programs. The experimental results provide insightful and comprehensive guidance for mutation testing of WS-BPEL programs in practice. In particular, our work is the systematic study in the selection of effective mutation operators specifically for WS-BPEL programs.