Nowadays many collaborative organizations require advanced semantic interoperability tools to enable cooperation and communication across distributed Web Information Systems (WIS). Adopting the service oriented technology, they have improved interoperability at the application level by using services for WIS functionalities provisioning. Furthermore, in order to support effective peer-to-peer collaboration, they require semantic interoperability techniques for service discovery and sharing. In this paper, we focus on semantic interoperability issues for distributed collaboration and provide techniques for building a service semantic overlay, across heterogeneous WIS. In particular, semantic links among peers that offer comparable services in a given domain are defined and maintained over the time. The service semantic overlay is ontologybased for formally organizing the shared services to enhance the capability of service interchange and interoperation among the collaborative system...