Water-soluble a-PHB oligomers (from dimer to dodecamer) were synthesized by anionic oligomerization of (R,S)-beta-butyrolactone and characterized by electrospray ionization tandem mass spectrometry (ESI-MSn). The oligomers were analogous to the degradation products remaining after enzymatic hydrolysis of synthetic high molecular weight atactic poly[(R,S)-3-hydroxybutyrate] by extracellular PHB depolymerases. Selected bacterial strains were used to test bioassimilability of a-PHB oligomers. It was found that not only two PHB-degrading bacteria (Alcaligenes faecalis T1 and Comamonas sp.) but also a non-PHB-degrading bacterium (Ralstonia eutropha H16) could grow on a-PHB oligomers as sole source of carbon and energy. Utilization of a-PHB oligomers by the three bacterial strains (total oligomer consumption and molecular weight distribution changes) was investigated by ESI-MS. Total oligomer consumption (tested after 30 h of bacterial growth) followed the same trend as the observed bacterial growth (A. faecalis > Comamonas sp. > R. eutropha). Mineralization of a-PHB oligomers demonstrates total biodegradability of synthetic high molecular weight atactic poly[(R,S)-3-hydroxybutyrate].