To further discuss the effect of zinc catalyst on diethylene glycol (DEG) formation in the preparation of poly(ethylene terephthalate) (PET), this research focused on the kinetics of DEG formation during PET synthesis from purified bishydroxyethyl terephthalate (BHET) monomer with zinc catalyst. The rate expression of DEG formation from purified BHET monomer and zinc catalyst is described. It was found that the activation energy of BHET with zinc catalyst in DEG formation during PET synthesis is lower than that of BHET without the addition of catalyst. Moreover, a comparison of zinc catalyst in DEG formation with antimony catalyst at the same concentrations showed that the reaction rate of BHET with zinc catalyst in DEG formation is greater than that of BHET with antimony catalyst. (C) 2000 John Wiley & Sons, Inc.