A mathematical model of a polycondensation reactor that can be used in the final stage for poly(ethylene terephthalate) (PET) is established and compared with experimental data obtained from a laboratory scale reactor with film renewal. Detailed side reactions are considered along with the diffusional removal of the small molecules through thin film. Among several kinetic constants, the polycondensation reaction rate constant k1(= k8) and diester group degradation reaction rate constant k7 have an influence over the degree of polymerization. The values of k1(= k8) and k7 for 0.05 wt% Sb2O3 were obtained as curve-fit values : (1) k1(+ k8) = 3.4 X 10(6) exp(- 18,500/RT’) (L/mol-min); (2) k7 = 1.3 x 10(11) exp(- 37,800/RT’) (min-1). Effects of the film exposure time, reduced pressure of vacuum. temperature, the initial terephthalic acid (TPA)/ethylene glycol (EG) mole ratio. the initial degree of polymerization, and catalysts were well explained by the model.