The problems of accessing and integrating heterogeneous information sources are becoming centerstage problems. One problem arising from accessing heterogeneous sources is semantic heterogeneity. In this paper, we propose a metadata dictionary based on domain ontology as an assistant mechanism for query processing the heterogeneous sources and resolving semantic heterogeneity. An XML-based data model is employed to manipulate and express the metadata dictionary contents. The inherent flexibility of XML technology enables system-wide interoperability suitable for a Web-based operations.