Abstract : Research in ergonomics and cognitive psychology point out difficulties in learning GIS (Medyckyj-Scott 1991, Nyerges 1991, Davies 1995 et, en France, Wurtz 1996). Our research is a follow-up of this work. It attempts to highlight the difficulties of learning GIS and possible ways exceed them, by understanding the mechanisms of learning GIS in the conditions offered by the CVT (continuous vocational training). We filmed and analyzed two GIS training sessions (approx. 30 hours each). Taken place three years apart, in the same training center, the training sessions have been conducted by the same two trainers. As Aline Robert (2008), we observed and recorded training activity without interfering: a fixed camera was placed in the gap, to cover the widest possible the occupied part of the classroom.On a theoretical level, our central thesis is the possibility and relevance to access processes and mechanisms of learning through mental representations built by adult learners. We relied on the theory of functional representation (Leplat), also called operative image (Ochanine) or representation for the action (Teiger, Weill-Fassina, Rabardel). Several other theoretical frameworks have also been mobilized: theories of adults learning (Barber, Bourgeois, Kaddouri), theories of vocational training (Mayen, Pastré, Vergnaud), the theory of instruments and instrumented action (Rabardel), the theory of mediation and guidance by others (Galperin, Savoyant). We reported on the learning process in its development through the evolution of the system of representation of people, which we had access through the analysis of verbal activity. Indeed, in verbal interaction between instructors and learners or between learners, representations of people emerge as resources (representations built before training) or as effects of training (under construction or processing representations).The evolution of the system of representations is systematically related to the conditions that determine it. The organization of the training session allows everyone to express themselves and underscores its representations. Emphasis is however placed on verbal interaction between trainers and learners and the effects produced. The formative function is provided by two trainers who are experienced professionals and who act simultaneous. This offers an interesting field of analysis of sharing transmission activity. We observe how trainers organize their joint intervention: they complement and support each other, but also contradict and prevent.Our thesis is part of a new orientation of vocational didactics theoretical field. This orientation has initiated by Mayen and consists to analyze training to work and not only the work for training. By analyzing the learning process, the thesis remains focused on work process. First, because the subject of learning is a professional instrument (GIS). Second, because trainees are professionals who do not separate their work joining the training session (Pastré, Mayen and Vergnaud, 2007). Finally, because the work situations and tasks experienced and verbalized by everyone are so many resources to learn during the training session.