The sequence distribution and the crystal structure of copolyesters synthesized from ethylene glycol, 1,3-propanediol, and dimethyl terephthalate with different molar volume ratios were investigated in this study. The triad sequence probabilities of ethylene/trimethylene terephthalate were characterized from the aromatic quaternary carbons by C-13 NMR. The composition of the copolyesters was determined from the aromatic quaternary carbons by C-13 NMR, and the methylene protons by H-1 NMR. Results show that 1,3-propanediol reacted faster with terephthalic acid in copolyester polymerization than ethylene glycol. The difference in monomer reactivity is significant in the polymerization. Although the constitutional units revealed a random distribution in the molecular chain by C-13 NMR, crystallites formed across the full range of ethylene glycol/1,3-propanediol composition by use of differential scanning calorimetry, a hot stage polarizing microscope, and a wide angle X-ray diffraction method. The WAXD deconvolution results show that the major constitutional repeating unit in the molecular chain dominates the crystal structure as a host crystal. The crystal structure was examined by a scanning electron microscope after a solvent etching. Photomicrographs show that the random distribution of the third constitutional unit in the molecular chain of copolyester significantly disturbs the host crystal formation and lamellar orientation. (c) 2005 Elsevier Ltd. All rights reserved.