The melt crystallization behaviors and crystalline structures of poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate), and poly(ethylene-co-trimethylene terephthalate) (PETT) were investigated with differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction at various crystallization temperatures (T(c)s). The PETT copolymers were synthesized via the polycondensation of terephthalate with ethylene glycol and trimethylene glycol (TG) in various compositions. The copolymers with 69.0 mol % or more TG or 31.0 mol % or less TG were crystallizable, but the other copolymers containing 34-56 mol % TG were amorphous. The DSC isothermal results revealed that the addition of a small amount of flexible TG (up to 21 mol %) to the PET structure slightly reduced the formation of three-dimensional spherulites. A greater TG concentration (91-100%) in the copolyesters changed the crystal growth from two-dimensional to three-dimensional. The DSC heating scans after the completion of isothermal crystallization at various T(c)s showed three melting endotherms for PET, PETT-88, PETT-84, and PETT-79 and four melting endotherms for PETT-9 and PETT. The presence of an additional melting endotherm could be attributed to the melting of thinner and imperfect copolyester crystallites. Analyses of the Lauritzen-Hoffman equation demonstrated that PETT-88 had the highest values of the product of the lateral and folding surface free energies, and this suggested that the addition of small amounts of flexible trimethylene terephthalate segments to PET disturbed chain regularity, thus increasing molecular chain mobility. (C) 2004 Wiley Periodicals, Inc.