Web service composition can be adopted to develop information systems through integration of services to obtain complex composed services. While interfaces of services are known at composition time, the quality of a composed service may depend on the ability of its component services to react to unforeseen situations, such as data quality problems and service coordination problems. In this work, we propose an approach to analyze the quality of composed services using fault injection techniques, by inspecting the reaction of a composed process to injected faults; the aim is to assess the process quality in terms of fault monitoring and, more generally, fault tolerance capabilities. The component services are analyzed either as black-boxes, when only input and output messages are considered or as white-boxes, when data sources used by services are considered. A test bed is illustrated on a selected example, and results of extensive testing are discussed and framed into a process analysis methodology.