Diffusion coefficients of ethylene glycol (EG) have been measured in poly (ethylene terephthalate) (PET) melts by a quartz-spring sorption apparatus. A simple mathematical model was developed to investigate the sorption behavior accompanied by chemical reactions of EG and PET at high temperatures. Diffusion coefficients are deduced from experimental data for an asymptotically thin sample in order to minimize the effects of reactions. The diffusion coefficient of EG is strongly dependent on the vapor pressure of EG and temperature but not on the molecular weight of PET in this experimental range (degree of polymerization 80-120). The diffusion coefficient of EG in PET melt at 265-degrees-C is 2.58 X 10(-7) cm2/s at the limit of zero concentration of EG. The activation energy for diffusion is 38.4 kcal/gmol, and the heat of solution for sorption is -44.9 kcal/gmol. The concentrations of the volatile materials resulting from reactions in PET-EG system were analyzed with gas chromatography. In addition, a fit of the current model to experimental data yields frequency factors for the polymerization reaction (k1) and the acetaldehyde formation reaction (k2) to be 5.84 X 10(8) cm3 / mol . min and 3.90 X 10(11) min-1, respectively.