The compositions of a series of copolyesters of ethylene- and trimethylene- terephthalate were determined by NMR. The values of randomness parameter revealed these copolyesters were random copolymers with average-number sequence lengths varied from 1 to 10. These copolyesters were investigated using temperature-modulated differential scanning calorimeter (TMDSC) and wide angle X-ray diffractometer (WAXD). The crystallization behavior was evidenced from the non-isothermal DSC thermograms and WAXD patterns. The crystallization rate and the intensity of diffraction peaks decrease with increasing the minor co-monomer content. The copolyester with equal amounts of monomers is still crystallizable even though the average sequence length is only 2.0, and its WAXD pattern is quite different from those of homopolymers. In the melting study, three distinct peaks of reversing melting were observed. Two peaks in the high temperature side are due to remelting of the recrystallized crystals formed during the heating scan, whereas the highest temperature peak is attributed to the melting of the crystals that are formed in relatively high temperature. The results of WAXD and TMDSC support the mechanism of melting-recrystallization-remelting for the multiple melting behaviors. (c) 2005 Elsevier Ltd. All rights reserved.