Poly(trimethylene terephthalate-co-2-methylethylene terephthalate) random copolymers of various compositions were synthesized via traditional two-step polycondensation by incorporating of 1,2-propanediol. The molar composition of trimethylene terephthalate and 2-methyl-ethylene terephthalate units and chemical structure were confirmed by means of H-1-NMR and Fourier transform infrared. The thermal properties of the copolyesters were evaluated by DSC and TGA. As far as the thermal properties is concerned, the main effect of incorporation of 1,2-propanediol was a lowering in the melting temperature, and an increment of glass transition temperature compared to homopolymer PTT. Due to the effect of the lateral methyl groups in the polymeric chain, the thermal stability is slightly decreased as the amount of the MET units is increased. Furthermore, the crystals of PTT/MET copolyesters were observed by hot-stage optical polarizing microscopy at the indicated crystallization temperatures. As expected, the incorporation of MET units in the polymer chain of PTT was found to decrease the dimension of the crystals. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3419-3426, 2010