Internet services can now be used from mobile terminals. The main standard supporting this technology, WAP, will enable new services since it is compatible with network technologies like IP and UMTS. In parallel, powerful methods must be proposed to validate the underlying protocols in order to guaratee reliability and interoperability of new products. Our work, based on formal methods, contributes to WAP testing efforts by proposing an approach to the development of interoperability tests. We illustrate this approach with the design of tests suites for the WSP-protocol operating over a WAP transaction service.