This paper presents the methodology and the results of a duration study performed for French and motivated by the construction of a French text-to-speech system [7]. Steps necessary for the construction of a statistically based duration system are described, with special attention on specific aspects of French, such as liaison and schwa deletion. The scientific contribution of this research is to add acoustic evidence to the knowledge of the complex effects of contextual duration phenomena on phonemic duration in French, extending perceptually determined knowledge.