A method for analysing the evolution of the conceptual framework of students during an experimental research work is presented. It has been used on chemistry students during their master's degree final project considered as an open problem solving activity. The fist results reveal important modifications of the conceptuel networks of the students during this activity which may last months. At the beginning, the basic concepts are weakly connected to other notions; at the end, they are much more connected, which reveals an enrichment even towards the experimental practice.