Abstract : Testing Fault Tolerance Mechanisms (FTM's) is crucial for the development of today's Web Service applications. In this work, we propose a methodology for assessing the efficacy of FTMs applied to Web services applications distributed over the Internet. We present a tool that uses application level fault injection techniques to inject communication faults by using a network Emulator Service. The emulator also generates additional workload on the tested system in order to produce more realistic results. As well as allowing the user to generate a fault model script, the tool provides, (by analyzing WSDL documents), the ability to inject selected faults into the exchanged SOAP messages. The tool can be used to test either a single Web Service or to test composed services without any modification to the system under test.