A series of blends of bacterial poly(R-hydroxybutyrate) (PHB), with different miscible amorphous components, were prepared. The molecular mass of the amorphous components was varied from M-n approximate to 600 to 200,000 g/mol. Several factors were found to influence the stress-strain behavior of PHB blends: the glass-transition temperature, the average molecular mass of amorphous phase, the content of PHB in the blend: the crystallinity, and the drawing temperature. It was found that a high extension ratio at rupture lambda(R) > 3 is obtained only if the PHB content is less than 60% (crystallinity X-c < 0.4), although this holds only for blends of bacterial PHB with a high molecular mass (M-n > 30,000) amorphous component. The lowering of glass-transition temperature by the addition of low molecular mass additives (plasticizers) deteriorates the tensile properties. Measurements at elevated temperatures of ductile blends yielded a marked decrease of both stress at rupture sigma(R) and extension ratio at rupture lambda(R). (C) 2004 Wiley Periodicals, Inc.