The phase behaviour of a series of blends containing poly(hydroxybutyrate) with poly(hydroxybutyrate-co-hydroxyvalerate) or a mixture of two copolymers of differing hydroxyvalerate (HV) content has been investigated using calorimetric methods backed up by morphological observations. Under static conditions, liquid-liquid phase separation (LLPS) was found to occur in blends where the relative HV level (i.e. difference in HV between the blend and the component of lowest HV) exceeded about 12%. The region of LLPS is asymmetrically placed at the high HV side of the phase diagram and the extent of phase separation increases as the relative HV level between the blend components is increased. Phase separation was promoted by shear. The results are compared with similar behaviour found in blends of linear with branched polyethylene.