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Verifying whether a service implementation is conforming to its service-level agreements is important to inspire confidence in services in a service-oriented architecture (SoA). Functional agreements can be checked by observing the published interface of the service, but other agreements that are more non-functional in nature, are often verified by deploying a monitor that observes the execution of the service implementation. A problem is that such a monitor must execute in an untrusted environment. Thus, integrity of the results reported by such a monitor crucially depends on its integrity. We contribute an extension of the traditional SoA, based on hardware-based root of trust, that allows clients, brokers and providers to negotiate and validate the integrity of a requirements monitor executing in an untrusted environment. We make two basic claims: first, that it is feasible to realize our approach using existing hardware and software solutions, and second, that integrity verification can be done at a relatively small overhead. To evaluate feasibility, we have realized our approach using current software and hardware solutions. To measure overhead, we have conducted a case study using a collection of Web service implementations available with Apache Axis implementation.