In this paper, we present a semantic based approach for integrating heterogeneous data sources, and we implement our approach on the scientific domain of Cultural Heritage Conservation-Restoration. First, we evaluate the context and the current needs; the observations confirmed the critical need of a data integration system taking advantage of semantics. Second, we describe the process that we followed in order to build an ontology dedicated to provide a unified understanding of the data. Third, we describe the ontology-based mediator system designed for effectively querying the heterogeneous distributed sources.