Service-oriented computing (SoC) promotes a paradigm where enterprise applications can be transformed into reusable, network-accessible software modules or services (i.e. Web services). In many cases, existing concrete applications can be wrapped to perform within the SoC environment by (1) converting their required input data and output provisions into XML-based messages (e.g. SOAP) and (2) specifying the newly-created services using other XML-based software specifications (e.g. WSDL). In addition, enterprise organizations also devise natural language specifications to describe the service capability. Unfortunately, consumers of these capabilities often misinterpret the data requirements for using the underlying services. In this paper, we propose a generic model for data-centric Web Services that aids formal specification of service-data interactions and provides practical and verifiable solutions to eliminate data ambiguity and promote service reusability.