To explore the possibility of using the partial pyrolysis of microbial poly(hydroxybutyrates) (PHBs) for obtaining single 3HB, 4HB, and mixed 3HB/4HB oligomers, the pyrolysis of P(3HB), P(4HB), and P(3HB-co-4KB) was performed and the products obtained were analyzed by HPLC and fast atom bombardment mass spectrometry (FAB-MS). Our results show that the partial pyrolysis of P(3HB) and P(4HB), followed by HPLC separation of the products, affords a suitable route for the synthesis of linear 3HB oligomers and of 4HB oligolides, respectively. The thermal degradation of P(3HB), P(4HB), and P(3HB-co-4HB) copolymers containing 3-34% of 3HB units was investigated by using several techniques : monitoring the time-dependent changes in molecular weight of melt samples in the range 180-200 degrees C; direct pyrolysis mass spectrometry; preparative pyrolysis and subsequent IR, NMR, HPLC, and FAB-MS analysis of the pyrolysate. Contrary to the claim that the rate constant (K-d) Of chain scission is not influenced by the composition of the copolyesters, our results show that the rate of chain scission is influenced by the composition of copolyesters and that a mixture of cyclic and open chain oligomers is formed in the pyrolysis process. These findings indicate that P(3HB-co-4HB) copolymers undergo thermal degradation by two competitive random chain scission mechanisms : a beta-CH hydrogen transfer at the 3HB units and an ester exchange cyclization at the 4HB units.