The thermal degradation of poly(lactic acid) has been studied using thermal gravimetric analysis over the temperature range 300-700 K, and following a sterilization dose of 50 kGy under vacuum, nitrogen or air. Isothermal weight loss studies have been found to follow complex kinetics with activation energies determined by the MacCallum method for various fractional conversions in the range 72-103 kJ mol(-1). Dynamic thermogravimetric analysis at heating rates from 10 to 25 K min(-1) showed the maximum rate of weight loss occurs in the range 550-650 K with the onset of degradation being in the range 450-500 K. The thermograms observed for the unirradiated polymer under a nitrogen atmosphere showed a very small shoulder on the low temperature side of the thermograms. The Kissinger expression, which is based on the temperature for the maximum rare of weight loss, yielded an activation energy of 109 kJ mol(-1) for thermal degradation under nitrogen and 131 kJ mol(-1) under oxygen. Compared with those of unirradiated samples, the weight-loss profiles under nitrogen for samples irradiated under vacuum or nitrogen were shifted towards higher temperatures, but the profiles for the samples irradiated in air did not change significantly. The shift to higher temperatures for the former two samples was attributed to modification of chain-end hydroxyl groups. (C) 1999 Society of Chemical industry.