Abstract The miscibility of binary blends of bacterial poly[( R)-3-hydroxybutyric acid] /P[( R)-3HB] n0 ⊮ M w = 650 000) with poly[( S)-lactic acid] /P[( S)-LA] n0 ⊮f various molecular weights ( M w = 9900-530 000) have been studied by means of differential scanning calorimetry (d.s.c.). D.s.c. analysis revealed that the structure of P[( R)-3HB]/P[( S)-LA] blends was strongly dependent on the molecular weight of the P[( S)-LA] component. The blends of P[( R)-3HB] with P[( S)-LA] of high M w values over 20 000 showed two phases in the melt at 200°C, while the blends of P[( R)-3HB] with P[( S)-LA] of low M w values below 18 000 were miscible in the melt over the whole composition range. On the basis of the relationship between the miscibility of blends and the molecular weight of the P[( S)-LA] component, the difference in the solubility parameters δ 1 and δ 2 of the blend components in the Flory-Huggins equation was estimated to be 0.34 ( J cm −3) 1 2 .