The study presented in this paper has been carried out in the framework of phonological variability in French, with applications in automatic speech processing in mind. The specific aim of our study is to model inter-speaker phonemic variants at word boundaries: global strategies and strategies specific to different classes of speakers. Data have been collected from casual speech corpus. Variants are defined through their relationship with the prosodic structure of a given utterance, This structure depends on syntactic organization of discourse. One example of this phenomena is given here with the pronunciation of the schwa at words' frontiers.