This article describes an empirical study concerning the mastering of the chemical equilibrium concept by prospective physical sciences teachers. The main objective was to check whether the concept of chemical equilibrium had become an integrating and unifying concept for them, that is to say an operational and functional knowledge to explain and predict chemical phenomena involving chemical equilibrium. A quantitative survey was conducted among students from various teacher training institutes of universities in France. We have tried to evaluate the preferred types of discourse used by these students for interpreting experimental situations involving chemical equilibria, to what extent they are aware of the dynamic aspect of the equilibrium state, and to how far their experimental knowledge has been improved by this concept. The results show that the concept of chemical equilibrium has not acquired the status of an integrating and unifying concept for the majority of prospective physical sciences teachers. Few of them master the concept of chemical equilibrium in a systematic and efficient way. They conceptualize chemical phenomena involving chemical equilibrium only through the chemical reaction concept. Moreover, experimental knowledge is partly dissociated from theoretical knowledge. (Contains 2 tables, 1 figure, and 1 note.)