The authors report on an experimental study on the differences between spoken and written queries. A set of written and spontaneous spoken queries are generated by users from written topics. These two sets of queries are compared in qualitative terms and in terms of their retrieval effectiveness. Written and spoken queries are compared in terms of length, duration, and part of speech. In addition, assuming perfect transcription of the spoken queries, written and spoken queries are compared in terms of their aptitude to describe relevant documents. The retrieval effectiveness of spoken and written queries is compared using three different information retrieval models. The results show that using speech to formulate one's information need provides a way to express it more naturally and encourages the formulation of longer queries. Despite that, longer spoken queries do not seem to significantly improve retrieval effectiveness compared with written queries.