Poly(ethylene/butylene terephthalate) (EB) copolyesters were synthesized from ethylene glycol (EG), 1,4-butanediol (BDO), and dimethyl terephthalate with different molar volume ratios. The constitutional units, characterized by C-13 NMR, revealed a random distribution in the molecular chain. Nevertheless, crystallites formed across the full range of EG/BDO composition can be observed with the use of a differential scanning calorimeter and a wide angle X-ray diffraction method. The reflection profile of the EB copolyesters was deconvoluted. Also, in the poly(ethylene/trimethylene terephthalate) (ET) copolyesters previously studied, the crystal structure was dominated by the sequence distribution of the copolyesters. The crystal structures of EB and ET copolyesters, in comparison with the different constitutional repeating units in the molecular chain, are discussed in this study. The glass transition temperature of EB and ET copolyesters were analyzed, considering the copolymer composition and the sequencing of the monomers, and their values were compared to the Johnston's equation. In addition, the analysis of the melting temperatures of both the EB and the ET copolyesters was carried out. The equilibrium melting temperatures of both copolyesters were calculated according to the Hoffman-Weeks extrapolative method. (c) 2007 Wiley Periodicals, Inc.