Cocrystallization and phase segregation in blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHBV] and poly(3-hydroxybutyrate-co-3-hydroxypropionate) [PHBP] at 80-100 degreesC were analyzed through the in situ observation of crystallization by FTIR microscopy and the measurement of melting curve by DSC. To trace the crystallization process of the blend components separately by a FTIR microscope, deuterated PHBV (D-PHBV) is blended with normal PHBP. The C-D and C=O stretching bands of the IR spectra are used to trace the crystallization of D-PHBV and the whole blend, respectively. Five pairs of PHBV and PHBP were selected so as to make variations in the spherulite growth rates (G) of components and their difference. Three of them are the pairs of which PHBV has a higher G than PHBP. They show four types of crystalline phase structures with various extent of phase segregation: complete cocrystallization, cocrystallization forming PHBV-rich cocrystals, phase segregation forming PHBV-rich cocrystals and PHBP microcrystals, and phase segregation forming PHBV crystals and PHBP microcrystals. The extent of phase segregation depends on the G values and their difference. The lower G and the larger difference favor phase segregation. The other two blends are comprised of PHBP with a higher G value and PHBV with a lower G. Even if the G values and their difference fall within the range where the former three blends shows advanced phase segregation, the latter two blends show cocrystallization. This contradiction indicates another important factor, the compatibility of the monomers to the crystalline lattice of the other. PHBV shows isomorphous behavior; i.e., HV units can be partially included into the PHB lattice, while PHBP cannot. Therefore, any part of PHBV chains easily gets into these lamellas because the HV units in the chains can participate in the PHB-type crystalline lattice induced by PHBP. On the other hand, only the homogeneous sequences of HB units of PHBP can participate in the lamellas induced by PHBV.