The thermal degradation behaviour of bacterial poly (3-hydroxybutyrate) has been studied by Pyrolysis–GC–FTIR using a semi continuous furnace in the temperature range of 200–600°C. At temperatures lower than 400°C, 2-butenoic acid and higher degradation products have been obtained. However, at higher temperatures propene and carbon dioxide are the major degradation products. The relative composition of the detected compounds changes as a function of pyrolysis temperature. The application of Hi-Res TGA technique permits the minimum temperature of PHB decomposition to be determined with increased precision.