A thermodynamic simulation of the phase-separation process of an off-critical blend, based on a thermoplastic matrix with a reactive epoxy system undergoing polycondensation at a constant temperature, was performed. The model considered the composition dependence of the interaction parameter, chi(T, Phi(2)) (where T is the temperature and Phi(2) is the volume fraction of polystyrene), along with the polydispersity of both polymers. For every level of conversion, the simulation provided the amount, composition, stoichiometric ratio, and conversion of each phase present. The accuracy of the model was proved by the good agreement between the experimental and predicted glass-transition temperatures and heat capacity changes at the glass-transition temperatures for both phases. (C) 2004 Wiley Periodicals, Inc.