The rate constants of thermolysis of poly(tetramethylene terephthalate) as determined by different methods of evaluation were compared. Considerable agreement of the overall rate constant of thermolysis with the rate constant of random ester group scission was observed. The thermal stability of poly(tetramethylene terephthalate) is compared with that of other poly (alkylene terephthalates) and the values are related to the chemcial structure.