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 À1 . Dynamic thermogravimetric analysis at heating rates from 10 to 25 K min À1 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 rate of weight loss, yielded an activation energy of 109 kJ mol À1 for thermal degradation under nitrogen and 131 kJ mol À1 under oxygen. Compared with those of unirradiated samples, the weight-loss pro®les under nitrogen for samples irradiated under vacuum or nitrogen were shifted towards higher temperatures, but the pro®les for the samples irradiated in air did not change signi®cantly. The shift to higher temperatures for the former two samples was attributed to modi®cation of chain-end hydroxyl groups.