This paper reports results from a project which sought to investigate the influence of two types of expertise – the knowledge of the search domain and the experience of the Web search engines - on the use of a Web search engine, called Exalead, by a panel of students. Forty six students (twenty four undergraduated students in psychology and twenty two undergraduates in other disciplines) were asked to give correct answers to eight questions about definitions of psychology concepts, without any time constraint. Results show that participants with good knowledge in the domain on the one hand and participants with high experience of the Web on the other had the best performances. Participants with low experience of the Web showed less effectiveness than the other participants. Future research is proposed to know the best aids to users of information retrieval systems.