Solid structures of the compositionally fractionated bacterial poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s [P(3HB-co-3HP)], bearing much narrower comonomer compositional distributions, were studied by high-resolution solid-state C-13 n.m.r. H-1-C-13 cross-polarization relaxation behaviour (T-CH and T-lp(H)) of chain-carbon nuclei was found to strongly depend on the micro-environments they were involved in, and the resonance contributions pertaining to different phases were therefore distinguished. Employing CP/MAS and PST/MAS methods, C-13 resonance signals from the crystalline and non-crystalline regions were selectively enhanced. The observed chemical shifts demonstrated that co-crystallization did not occur in the P(3HB-co-3HP) system. Spin-lattice relaxation times (TF) of carbon nuclei attributed to 3HB and 3HP units were found to be considerably different, reflecting their distinguishable difference in local segmental dynamics. From the lineshape analyses of 3HB methyl carbon resonance measured by a DD/MAS method, it was revealed that the presence of minor 3HP comonomer units significantly suppressed the crystallization of a 3HB-rich semicrystalline copolyester. In addition, a correlation was found between average block length of 3HB sequences estimated by solution-state C-13 n.m.r. spectra and crystallizability of semi-crystalline copolyesters.