Currently, fault management in Web Services orchestrating multiple suppliers relies on a local analysis, that does not span across individual services, thus limiting the effectiveness of recovery strategies. We propose to address this limitation by employing ModelBased Diagnosis to enhance fault analysis. In our approach, a Diagnostic Web Service is added to the set of Web Services providing the overall service, and acts as a supervisor of their execution, by identifying anomalies and explaining them in terms of faults to be repaired. Categories and Subject Descriptors D.2.11 [Software Engineering]: Software Architectures General Terms Languages, Standardization Keywords Web Service composition, fault management, diagnosis