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Recent distributed systems, in a P2P scenario, are characterized by a set of independent peers that dynamically need to cooperate by sharing data and services. For effective collaboration, under highly dynamic conditions and in absence of a global view of the resources shared across the information systems, semantic modeling tools need be defined. In particular, the emergence of semantics is a key issue to enforce timely discovery and integration of distributed data and services. In the paper we discuss, in particular, the construction of a semantic overlay for service sharing and discovery in service-oriented applications in a P2P scenario. The semantic overlay is built over the P2P network: each peer has ontology-based service descriptions; semantic links between similar service descriptions belonging to different peers are established and maintained according to the network evolution; the result is P2P-integrated knowledge space (here considered for services but extendible to data). The semantic overlay can be seen as a continuously evolving conceptual map across collaborative peers that provide similar services and constitute synergic service centres in a given domain. The semantic links enable effective similarity-based service search and optimization strategies are defined for request propagation over the P2P network. From the conceptual modeling perspective, the semantic overlay can be considered as an evolved conceptual representation where specific modeling requirements due to new technologies, service-oriented technology and P2P technology, are considered.