In this paper, we study how the performance and usability of web dialogue systems could be enhanced by using an appropriate representation of the different types of knowledge involved in communication: general dialogue mechanisms, specific domainrestricted linguistic and conceptual knowledge and information on how well the communication process is doing. We describe the experiments carried out to analyze how to improve this knowledge representation in the web dialogue system we developed. Categories and Subject Descriptors H.5.2 [Information Interfaces and Presentation]: User Interfaces ? natural language, evaluation, user centered design. General Terms Design, Experimentation, Human Factors. Keywords Web dialogue systems, mixed-initiative dialogues, adaptive dialogue systems, evaluation.