Despite the industrial significance of poly(bisphenol A carbonate), there is a scarcity of open literature on the equilibrium of the melt-phase process. In fact, the equilibrium constant (K-eq) for this reaction has never been measured directly. This article describes a process on the basis of NMR for the measurement of K-eq for the reaction between bisphenol A and diphenyl carbonate in the presence and absence of a catalyst. The apparent enthalpy and entropy were calculated using a van't Hoff plot. Decomposition of bisphenol A is a common side reaction in the melt-phase reaction performed at high temperatures in the presence of catalyst. The effect of these side reactions on the K-eq in the presence of catalyst is determined.