Testing compositions of web services is complex, due to their distributed nature and asynchronous behaviour. However, research in this field is scarce. We propose a new testing method for compositions of web services. A formal verification tool (the SPIN model checker) will be used to automatically generate test suites for compositions specified in an industry standard language: BPEL. Adequacy criteria will be employed to define a systematic procedure to select the test cases. Preliminary results have been obtained using a transition coverage criterion.