Melt crystallization behaviors of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate-co-isophthalate) (PETI) containing 2 and 12 mol % of noncrystallizable isophthalate components were investigated. Differential scanning calorimetry (DSC) isothermal results revealed that the introduction of 2 mol % isophthalate into PET caused a change of the crystal growth process from a two-dimensional to a three-dimensional spherulitic growth. The addition of more isophthalate up to 12 mol % into the PET structure induced a change in the crystal growth from a three-dimensional to a two-dimensional crystal growth. DSC heating scans after completion of isothermal crystallization at various T-c's showed three melting endotherms for PET and four melting endotherms for PETI-2 and PETI-12. The presence of an additional melting endotherm is attributed to the melting of copolyester crystallite composed of ethylene glycol, tere-phthalate, and isophthalate (IPA) or the melting of molecular chains near IPA formed by melting the secondary crystallite T-m(I) and then recrystallizing during heating. Analyses of both Avrami and Lauritzen-Hoffman equations revealed that PETI containing 2 mol % of isophthalate had the highest Avrami exponent n, growth rate constant G(o), and product of lateral and end surface free energies sigma sigma(e).