The growing of the Internet has emphasized the need for interoperability among different and heterogeneous systems, i.e. their ability to exchange data and services in a transparent way. This article addresses interoperability in a GIS context where systems, historically monolithics, need to exchange spatial data. The work is based on the models proposed by the OpenGIS Consortium, whose mission is the unification of spatial data models and services in order to achieve interoperability among GISs. The paper faces the problems connected with the interoperability at different levels of abstraction. At the highest level (the spatial data on the Internet) a set of common interfaces are adopted to represent both data and services; at the bottom level (database level), the concept of a specialized server (called OGIS/SQL server) is introduced, which is the entity that tries to obtain data integration among the data sets. This paper discusses, also, an implementation of a JavaRMI-based application, that allows different and web distributed data sets to be interconnected. The architecture of the application is made up of two different relational databases (one stores rivers and the other cities), a distributed object (representing the origin of its own spatial reference system), and a client that acquires (by query) spatial data from the previous mentioned spatial data sources, stores them in a local database and then visualizes the result.