In this paper, we describe how domain ontologies are used in a dialogue system guiding the user to access web public administration contents. The current implementation of the system supports speech (through the telephone) and text mode in different languages (English, Spanish, Catalan and Italian). Categories and Subject Descriptors H.5.2. [Information Interfaces]: User Interfaces – Natural language, Voice I/O, User interface management systems (UIMS). General Terms: Management, Design, Human Factors.