New security challenges are arising from the widespread adoption of Web service technology. Indeed such technology does not directly address the problem of securing the information flow between the clients and the service providers. For this reason, many boards and technical committees are currently involved in research projects aimed to the definition of standards and specifications related to the provision of security properties. In this work, we propose a framework to accurately measure the number of accesses to a Web service, which can be invoked by authenticated clients only. The system we propose can be helpful in the development of new business models, where an audit agency is in charge of registering clients accessing the services, and of paying the servers which show a valid proof of the number of requests serviced.