The throughput and quality of work of the geohydrologist can substantially be increased through the introduction of a geohydrological information system. This is primarily a result of the fact that these systems integrate the different types of geohydrological data with tools for processing and analysis into one seamless working environment. Site characterization studies, during which information of many different origins has to be evaluated and combined, will benefit in particular from the use of such an integrated system. Due to the spatial character of geohydrological data and processing, the incorporation of a GIS into the geohydrological information system becomes a prerequisite. However, the integrated character of such systems will impose specific demands on the extensibility of any candidate GIS. The application of object-oriented principles to GIS improves their extensibility and allows for an almost seamless integration with geohydrological applications. This paper demonstrates how object-oriented concepts can be used to extend a GIS beyond its standard functional domain.