The relation between thermal degradation and carboxyl content of polyethylene tereph-thalate is investigated in the polycondensation process. It is found that in the presence of butyl titanate or stannous oxalate as a polycondensation catalyst the carboxyl content is not always on the increase in all high vacuum stage. Three methods (TGA

[η] and COOH) for evaluating thermooxidative degradation rate constants are employed and the results are discussed. The effects of single or composite stabilizers and polycondensation catalysts on the thermooxidative degradation have also been studied.