This research focused on the kinetics of diethylene glycol (DEG) formation from the bishydroxyethyl terephthalate (BHET) monomer with a proton catalyst. In this study, the effect of proton concentration and of reaction temperature on DEG formation are discussed. Also, the rate equation of DEG formation from the BHET monomer with a proton catalyst is described. It was found that, as far as kinetics is concerned, the reactivity of the hydroxyl end groups of BHET with protons is greater than that of ethylene glycol (EG) with protons in DEG formation. In addition, the activation energy of BHET with protons is much lower than that of BHET with itself, that is, as protons emerge during the process of PET synthesis from BHET, they catalyze DEG formation. This study provides additional kinetics data to that described in our studies previously published (J Polym Sci Polym Chem Ed 1998, 36,3013; 1998, 36, 3081). (C) 2000 John Wiley & Sons, Inc.