We designed two experiments that tested the listeners' perceptual capacities during online segmentation of homophonic word boundaries while processing sentential information. In French, listeners often use variations in fine acoustic indices to detect word beginnings. We measured event-related potentials (ERPs) evoked by phonemically identical sequences, such as l'affiche ("the poster") and la fiche ("the sheet"), both [lafiʃ], which were contained in either congruent or incongruent sentences. Results showed that although listeners can detect acoustic variations in homophonic sequences, these may not be salient enough when contextual information is also present. Shifting attention from sentence meaning (Task 1) to lexical information (Task 2), enhanced the listeners' perception of fine-grained acoustic details. Thus, topdown processes are likely to modulate speech perception and segmentation.