The feasibility of the on-line control of emulsion terpolymers based on calorimetric measurements was assessed. First the inference of the terpolymer composition from calorimetric measurements was studied and experimentally validated. Two control strategies were used to maximize the production of BuA/MMA/Sty latexes under safe conditions, maintaining simultaneously the terpolymer composition of the polymer at predefined values. The application of these strategies to systems affected by a severe gel effect is discussed.