Solid-state structures and crystallization kinetics were compared between poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHB-HV] and PHB/PHB-HV blends exhibiting the cocrystallization. As cocrystallizable blends, both the blends showing complete cocrystallization, i.e. the PHB content in the crystalline phase is the same as that of the whole blends, and the blends forming a PHB-rich crystalline phase were used. The PHB and HV content in the cocrystalline phase were determined by high-resolution solid-state C-13 NMR spectroscopy. In order to determine these contents with a minimum experimental error, site-specific C-13-labeled PHB/PHB-HV blends and PHB-HV copolymers were used. The crystallinity, lamellar structures, spherulite growth rate, and melting behavior were analyzed by wide-angle X-ray diffraction, small-angle X-ray scattering, polarized microscope, and differential scanning calorimetry, respectively. In these data, no difference was observed between the complete-cocrystallizable PHB/PHB-HV blends and the PHB-HV copolymers with the same overall HV content. On the other hand. the PHB/PHB-HV blends forming a PHB-rich crystalline phase has the amorphous layers thicker than that of the PHB-HV copolymers with the same overall RV content. Based on the collected data, the similarity and differences in the solid-state structures and properties between PHB-HV copolymers and cocrystallizable PHB/PHB-HV blends were discussed. (C) 2004 Elsevier Ltd. All rights reserved.