The effect of thermodynamic phase equilibrium on the kinetics of semibatch melt polycondensation of bisphenol-A and diphenyl carbonate was studied for the synthesis of polycarbonate. In the melt-polymerization process, a partial loss of diphenyl carbonate occurs as the reaction by-product phenol is removed from the reactor. To obtain a high molecular weight polymer under high temperature and low-pressure conditions, a stoichiometric mol ratio of the two reactive end groups needs to be maintained during the polymerization. In this work, vapor-liquid equilibrium data for a binary mixture of phenol and diphenyl carbonate are reported and they are used in conjunction with the Wilson equation to calculate the exact amounts of diphenyl carbonate and phenol returned from a reflux column to the reactor. A good agreement between the reactor model simulations and the experimental polymerization data was obtained. It was also observed that diphenyl carbonate is quickly consumed during the early stage of polymerization and the fraction of evaporated diphenyl carbonate refluxed to the reactor is essentially constant during this period.