Abstract Questionnaires are nowadays widely used usability evaluation instruments, and several generic usability questionnaires are available. But these generic artifacts are not always appropriate to evaluate a given setting, and constructing a questionnaire from scratch is a complex task requiring both expertise and resources, so that discount-usability approaches to questionnaire-based evaluation can make a good option in many cases. In this work, a novel knowledge-based approach to design Web usability questionnaires is described. The questionnaire model comprises different ontologies including concepts regarding questions and questionnaires, the different measures that can be obtained and the tasks that have to be carried out by users in order to evaluate a specific kind of Web application. As a proof of concept for the model, a prototype questionnaire design application is also described. The application demonstrates how facts can be gathered through a guided dialogue with the us...