The miscibility and morphology of blends of biodegradable poly(3-hydroxybutyrate) (PHB) and poly(vinyl butyral) (PVB) were studied by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). PHB is a semi-crystalline polymer while PVB can be regarded as a random copolymer containing vinyl butyral (VB) and vinyl alcohol (VA) units whose composition ratio can be systematically changed over a wide range. DSC measurements show two T(g)s in the amorphous phase for all PVB and PHB blends, indicating phase immiscibility. However, partial miscibility was observed in 50/50 w/w blends containing PVBs with 25-36 wt% VA content, demonstrated by the inward shift of the T(g)s of the two phases relative to those of the neat components. This shift exhibits a maximum value around 31 wt% of VA in PVB. The segmental interaction parameters chi (VA,HB), chi (VB,HB) and chi (VA,VB) were determined and the copolymer composition dependence of the overall interaction parameter between PVB and PHB, chi (12), was then calculated. This parameter shows a minimum around 31 wt% VA content, consistent with the experimental data. The presence of a minimum can be interpreted in terms of a copolymer effect in which the unfavorable interaction between PVB and PHB is minimized at a certain copolymer compositions due to the repulsive intramacromolecular interaction between VA and VB units within the PVB chains. The composition manifesting minimum immiscibility was also consistent with the melting temperature (T-m) depression and a minimum in the fractional crystallinity of the PHB phase in the blends around this VA content. A co-continuous morphology was observed in the blends containing 25-33 wt% VA content PVBs, which might well result from more favorable interactions between the two component polymers at this copolymer composition.