The thermal degradation of poly(lactic acid) (PLA) and poly(β- hydroxybutyric acid) (PHB) was investigated by means of several thermoanalytical methods, including thermogravimetry, differential scanning calorimetry, pyrolysis-MS and pyrolysis-GC-MS. These methods supplied information on the kinetics of the polymer decomposition as well as on the product composition. The analysis of LA-oligomers provided new insights into the stereo-chemistry of the degradation. The results are discussed in view of the known reaction mechanisms for ester pyrolysis. Some of the present results cannot be explained on this basis.