AbstractEster interchange equilibrium products prepared from dimethyl terephthalate and ethylene glycol, in the presence of zinc acetate as a catalyst, have been investigated at 180 and 195°. For the analysis of these products quantitative volumetric methods for the determination of free methanol and of free ethylene glycol have been developed. In addition, a method for the fractional vacuum distillation and sublimation of methanol, ethylene glycol, dimethyl terephthalate, and β‐hydroxyethyl methyl terephthalate has been worked out.The analytical results are evaluated in terms of equilibrium constants for the various reversible reactions pertaining to the present system. All equilibrium constants are found to depend only slightly on temperature and composition. The small values of the equilibrium constants related to the removal of methyl ester groups are in line with the practical difficulty of achieving complete glycolysis of dimethyl terephthalate before polycondensation to polyethylene terephthalate starts.The most important result is the fact that the “molecular” equilibrium constant KIof the first step of the glycolysis of dimethyl terephthalate equals the corresponding overall equilibrium constant Kein terms of functional groups after correction for the functionality of reactants and products. Prom this fact it is concluded that the “molecular” equilibrium constants KIand KIIfor the first and second step of the glycolysi