This paper describes the development of the functional specification for messages needed to support e-procurement interoperability across the UK public sector. The scope of the project is the procurement cycle from first enquiry to payment, between government buyers and third party suppliers. The methodology is based on the Office of the e-Envoy's e-service development framework, using a single UML model. The use of a single model has ensured coherence, while allowing the domain to be partitioned vertically using packages and horizontally by use cases. Packages include party, document, line, item and financial. 15 transactions (use cases) have been defined. This process-driven approach, covering the whole procurement cycle from beginning to end is somewhat different from other efforts which have focussed primarily on particular transactions such as purchase orders or invoicing. The use of a single model ensures a coherent structure that is necessary to support the re-use of data throughout the procurement cycle, such as the three-way match between purchase order, fulfillment notification and invoice, before authorising payment. Automating the three-way match, which is the core role of the accounts payable section, is one of the keys to realising productivity benefits for the buyer organisation. The overall specification is comprehensive, but a simple sub-set - "vanilla" purchase order and invoice - has been developed to meet the minimum needs of public sector buyers.