This article describes a synchronous teaching/learning exchange between Masters degree students in teaching French as a foreign language at the University of Lyon 2 and intermediate-level French students at the University of California, Berkeley during the 2006-07 academic year. The desktop videoconferencing platform allowed face-to-face audiovisual communication as well as synchronous written chat. Based on observational and interview data, the study explores the impact of multimodality and synchronicity on learners' and tutors' responses to a series of eight online pedagogical modules and exchanges intended to develop students' linguistic ability and cultural knowledge. Analysis of the two corpora (French and American) shows a progressive adaptation to the particularities of desktop videoconferencing and the development of specific competencies : among the American learners of French, improvement in fluency and greater ease in taking the floor ; among the French teachers-in-training, development of a professional online persona and flexibility in their pedagogical stance.