Miscibility behavior of poly( S-hydroxybutyrate) [PHB]/poly (vinylidene chloride-co-acrylonitrile) [P(VDC-AN)] blends have been investigated by differential scanning calorimetry and optical microscopy. Each blend showed a single T-g, and a large melting point depression of PHB. All the blends containing more than 40% PHB showed linear spherulitic growth behavior and the growth rate decreased with P(VDC-AN) content. The interaction parameter chi(12), obtained from melting point depression analysis, gave the value of -0.267 for the PHB/P(VDC-AN) blends. All results presented in this article lead to the conclusion that PHB/P(VDC-AN) blends are completely miscible in all proportions from a thermodynamic viewpoint. The miscibility in these blends is ascribed to the specific molecular interaction involving the carbonyl groups of PHB.