Running title: Voice activity detection based on lip movementsThis paper presents a quantitative and comprehensive study of the lip movements of a given speaker in different speech / non speech contexts, with a particular focus on silences (i.e., when no sound is produced by the speaker). The aim is to characterize the relationship between "lip activity" and "speech activity", and then to use visual speech information as a Voice Activity Detector (VAD). To this aim, an original audio-visual corpus was recorded with two speakers involved in a face-to-face spontaneous dialog, although being in separate rooms. Each speaker communicated with the other using a microphone, a camera, a screen, and headphones. This system was used to capture separate audio stimuli for each speaker and to monitor each