The effect of Sb2O3, which is used as the main catalyst in the production of poly(ethylene terephthalate) (PET) by melt polymerization, was studied on solid-state postpolycondensation. It was observed that the catalyst works efficiently even in the solid state. The number-average molecular weight of a PET prepolymer with an Sb2O3 content of 2000 ppm was increased from 18,350 to 40,800 after heating at 210 degrees C for 8 h under vacuum (2-3 Pa), whereas when the catalyst was absent, the molecular weight of the same prepolymer under the same conditions was just increased to 22,500. The activation energies and the frequency factors of the esterification and ester interchange reactions, which take place simultaneously during solid-state postpolycondensation, were determined.