The influence of thermal history on the properties of commercial P(3HB-co-12%-3HV) was studied. Thermogravimetric analysis and differential scanning calorimetry revealed that the plasticizer evaporated at 140 degrees C or higher. The loss of plasticizer during thermal treatment at, 170 and 180 degrees C resulted in a slight increase of the melting temperature of the polymer. The processing time and temperature, as well as the cooling procedure influenced the thermal behaviour of the material. A decrease in molecular weight with time was found at the temperatures investigated and this significantly affected the mechanical properties of the polymer prepared at 180 and 200 degrees C. The rate constant k(d) of thermal degradation was slightly higher for samples during a shape-forming process in a Minimax apparatus than during a quiescent heating process (DSC) and its value increased with temperature. Limiting the processing at 170 degrees C to 2 min gave a material with useful properties but in creasing the residence time resulted in a decrease in strength, elongation at break, molecular weight and viscosity although it did not significantly influence the modulus of elasticity. Materials prepared at 180 or 200 degrees C were more brittle and longer residence times resulted in a deterioration of the mechanical properties.