This research examined acoustic-phonetic cues to word boundary location in French consonant clusters, and assessed their use in on-line lexical segmentation. Two word-spotting experiments manipulated the alignment between word targets and syllable boundaries. A perceptual cost of such misalignment was observed for obstruent-liquid clusters but not for /s/ + obstruent clusters. For the former clusters, the analysis of a corpus of utterances showed systematic variations in segment durations as a function of the lexical assignment of the pivotal consonant. We conclude that the availability of acoustic-phonetic cues to word boundary location in consonant clusters depends upon the cluster class. When available, these cues are exploited in on-line lexical segmentation of speech.