A series of poly(ethylene terephthalate-co-isophthalate) copolyesterscontaining up to 50%-mole of isophthalic units were prepared by polycondensationfrom ethylene terephthalate and ethylene isophthalate fractions of linear oligomerscontaining from 5 to 6 repeating units in average. The polyesters were obtained in goodyields and with high molecular weights. The microstructure of the copolyesters wasstudied as a function of reaction time by 13C-NMR showing that a random distribution ofthe comonomers was achieved since the earlier stages of polycondensation. Themelting temperature and enthalpy of the copolyesters decreased with the content ofisophthalic units so that copolyesters containing more than 25% of these units wereamorphous. Isothermal crystallization studies made on crystalline copolyestersrevealed that the crystallization rate of copolyesters decreased with the content inisophthalic units.