The degradation of poly[(R,S)-3-hydroxybutyrate], a-PHB, binary blends with natural PHB (n-PHB) and poly(L-lactic acid), PLLA, respectively, has been investigated in soil. In such a natural environment, a-PHB blend component was found to biodegrade. The degradation of a-PHB-containing blends proceeded faster than that of respective plain n-PHB and PLLA. The molecular weight decrease of the n-PHB component was higher, while the same rate of bioerosion of both components was observed for the a-PHB/n-PHB binary blend. For the a-PHB blend with PLLA, the weight loss was accompanied by blend composition changes and the decrease of a-PHB content. However, the PLLA molecular weight decrease was lower in the blend in comparison with the plain PLLA sample. The increase of the number of microorganisms particularly observed for the soil where binary blends were incubated indicates that microbial degradation of a-PHB takes place. The terrestrial plant growth test (cress and barley) demonstrates no environmental toxicity of the materials studied.