A series of copolyesters were prepared by the direct melt transesterification of poly(trimethylene terephthalate) (PIT) and poly(butylene succinate) (PBS). The sequential structure of the copolyesters was analyzed with proton nuclear magnetic resonance spectroscopy, and the randomness of the copolyesters was calculated to be approximately 0.8. The cocrystallization, thermal behavior, and spherulitic morphology were investigated. The melting points of the copolyesters showed a pseudo-eutectic behavior exhibiting isodimorphic cocrystallization. Wide-angle X-ray diffraction indicated that the copolyesters crystallized in PTT crystals when the butylene succinate (BS) unit content was less than 60% and in the PBS crystals when the BS unit content was greater than 70%. The mechanical properties of the copolyesters were greatly influenced by the sequence lengths of the aromatic and aliphatic units. The incorporation of BS units into the PTT structure led to a faster rate of degradation of the copolyesters because of the decrease in the aromatic sequence length and the increase in the aliphatic sequence length. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1297-1306, 2011