Poly(butylene succinate) (PBSu) and two PBSu-rich poly(butylene succinate-co-propylene succinate)s were studied. Copolyesters were characterized as random copolymers, based on C-13-NMR spectra. TGA-FTIR was used to monitor the degradation products at a heating rate of 5 degrees C/min under N-2. FTIR spectra revealed that the major products were anhydrides, which were formed following two cyclic intramolecular degradation mechanisms by the breaking of the weak O-CH2 bonds around succinate groups. Thermal stability at heating rates of 1, 3, 5, and 10 degrees C/min under N-2 was investigated using TGA. The model-free methods of the Friedman and Ozawa equations are useful for studying the activation energy of degradation in each period of mass loss. The results reveal that the random incorporation of minor propylene succinate units into PBSu did not markedly affect their thermal resistance. Two model-fitting mechanisms were used to determine the mass loss function f(alpha), the activation energy and the associated mechanism. The mechanism of autocatalysis nth-order, with f(alpha) = a(m)(1 - alpha)(n), fitted the experimental data much more closely than did the nth-order mechanism given by f(alpha) = (1 - alpha)(n). The obtained activation energy was used to estimate the failure temperature (T-f). The values of T-f for a mass loss of 5% and an endurance time of 60,000 h are 160.7, 155.5, and 159.3 degrees C for PBSu and two the copolyesters, respectively. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3610-3619, 2012