AbstractA systematic design procedure of the prepolymerization stage of a polycondensation process is presented. This procedure treats polycondensation processes as reactive separation processes and offers an alternative to design through simulations. Feasible flowsheets and operating conditions are developed by analyzing the phase behavior of polycondensation systems at thermodynamic equilibrium. Functional end groups and apparent mole fractions are used to represent reaction kinetics and equilibrium, and real mole fractions are used to represent phase equilibrium. Reactive phase diagrams are plotted in transformed mole‐fraction space to facilitate visualization of the phase behavior. Three examples are presented: manufacture of polyethylene terephthalate (PET) from dimethyl terephthalate (DMT) and ethylene glycol (EG), from terephthalic acid (TPA) and EG, and manufacture of nylon‐6,6. For these processes, the results of the approach agree excellently with detailed iterative performance simulations. The approach also reveals additional feasible process alternatives and corresponding operating conditions.