MASC (Manageable and Adaptive Service Compositions)1* is a policy-based middleware for monitoring and control of composite Web services execution. The monitorable requirements are specified in the WS-Policy4MASC language that extends WS-Policy by defining new types of monitoring and control policy assertions. This paper focuses on MASC monitoring capabilities to detect business exceptions and runtime faults. Our solutions are complementary to the existing approaches and provide: synchronous and asynchronous monitoring both at the SOAP messaging layer and the process orchestration layer, greater diversity of monitoring and control constructs, as well as the externalization of monitoring and adaptation actions from definitions of business processes. We implemented a MASC proof-of-concept prototype and evaluated it on monitoring and adaptation scenarios from a stock trading case study. Our performance studies indicate that MASC overhead and scalability are acceptable.