The glycolysis of poly(ethylene terephthalate) (PET) by diethyleneglycol (DEG) in the presence of acetate catalyst usually produces a dark color product. Various catalysts have been used, but only ZnSO(4)7H(2)O gives a bright yellowish liquid material. In addition, after the reaction the catalyst ZnSO(4)7H(2)O can be separated easily and reused. Under microwave irradiation, the PET glycolysis is more efficient in time, conversion, and color improvement. The reaction mixture was purified by treatment with 20% sodium chloride solution. The structure of the isolated products was confirmed by FTIR, NMR, and HPLC-MS. Condensate of the volatile material from the glycolysis reaction was also isolated and analyzed by GC-MS. The mechanisms of PET degradation by DEG and side product formation were proposed. The transesterification of PET with excess DEG was strongly competed by the degradation of DEG to form dioxane and acetaldehyde as main volatile side products and by ether formation; therefore, the using of excessive DEG was unfavorable for the PET glycolysis.