Information systems in different public agencies need to seamlessly collaborate to support the delivery of public services through a one-stop government portal. For such collaboration to be successful, the systems must be organizationally, semantically and technically interoperable. In this paper, we illustrate the need for semantic interoperability services in Electronic Government and present a solution – Semantic Interoperability Middleware (SIM) that provides such services. Three case studies are drawn from the context of the delivery of welfare benefits involving the collaboration of different public and private organizations. Each case presents a need that is addressed through a SIM service – Mediation, Validation and Discovery. The paper also presents the requirements and architecture of SIM and highlights how SIM services address generic semantic differences and associated conflicts.