A series of poly(ethylene terephthalate-co-isophthalate) copolyesters containing upto 50%-mole of isophthalic units were prepared by polycondensation from ethylene terephthalate and ethylene isophthalate fractions of linear oligomers containing from 5 to 6 repeating units in average. The polyesters were obtained in good yields and with high-molecular-weights. The microstructure of the copolyesters was studied as a function of reaction time by C-13-NMR showing that a random distribution of the comonomers was achieved since the earlier stages of polycondensation. The melting temperature and enthalpy of the copolyesters decreased with the content of isophthalic units so that copolyesters containing more than 25% of these units were amorphous. Isothermal crystallization studies made on crystalline copolyesters revealed that the crystallization rate of copolyesters decreased with the content in isophthalic units. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1823-1830, 2010