We obtained by microbial synthesis an almost pure poly(4-hydroxybutyrate) (P(4HB)) sample (i.e., a P(3HB-co-4HB) copolymer containing only 3% of 3HB units). The thermal degradation of this material was investigated using thermogravimetry, direct pyrolysis mass spectrometry and also by preparative pyrolysis and subsequent NMR analysis of the pyrolyzate. Previous studies(7) on P(3HB-co-4HB) copolyesters have reported that the thermal degradation process occurs by a beta-CH hydrogen transfer, similar to the well-known case of P(3HB). However, the latter work did not distinguish between the thermal degradation fate of 3HB and 4HB units. Our findings indicate that the thermal decomposition of P(4HB) yields a series of cyclic oligomers (i.e., gamma-butyrolactone and its higher homologs) by an intramolecular exchange mechanism. Detailed mass spectrometric and H-1-NMR evidence is presented to prove this point.