The ability to record and account for the usage of computational resources in a standardised way across different systems from multiple administrative domains is a precursor to widespread grid deployment and availability. We describe the implementation of a resource usage service (RUS), which is based on two emerging Global Grid Forum (GGF) standards, and uses Web services technology. We address the issue of security, which is essential since usage information is potentially sensitive and must be verifiably accurate. We show that useful robust functionality can be provided using current Web services standards and we thus avoid over-dependency on evolving grid frameworks and middleware. This work has been undertaken within the UK e-Science project Markets for Computational Services, and builds on scenarios contributed to the project by organisations that manage and sell computing resource as a business.