The competition between cocrystallization and phase segregation in blends of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PI-IB-I-IV) containing 9 and 15% HV has been studied by C-13 cross-polarization magic-angle sample spinning (CPMAS) NMR and DSC techniques. The PHB homopolyester samples with selectively C-13-enrrched methylene carbon (PHB*) were used. Assuming a simple two-phase model, the C-13 resonances were resolved into two peaks arising from the crystalline and amorphous phases. Owing to the C-13-enrichment of PHB*, the relative areas of the crystalline peaks for the methylene and the methine carbon resonances on C-13 CPMAS NMR spectra change depending on the PHB* content in the crystalline phase. By comparing the ratio of these areas for the blends with those for pure PHB* and PHB-HV, the composition in the crystalline phase of the blends was determined. For PHB*/PHB-9%HV blends, the composition in the crystalline phase is very similar to that in the blend. This means that almost perfect cocrystallization of PHB and PHB -9%HV occurs in these blends. For PHB*/PHB-15%HV blends, the PHB content in the crystalline phase is larger than that in the whole blend. The phase segregation precedes the crystallization in these blends. Therefore, the degree of phase segregation (or the percentage of PHB-HV that segregates from the growth front of crystals) changes depending on the HV content of PHB-HV. As the HV content increases, the copolymer content in the crystalline phase decreases.