AbstractA 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 Mn ≈ 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 λR > 3 is obtained only if the PHB content is less than 60% (crystallinity Xc < 0.4), although this holds only for blends of bacterial PHB with a high molecular mass (Mn > 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 σR and extension ratio at rupture λR. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2528–2537, 2004