The multiple melting behavior of poly(ethylene terephthalate) (PET) homopolymers of different molecular weights and its cyclohexylene dimethylene (PET/CT) copolymers was studied by time-resolved simultaneous small-angle X-ray scattering/wide-angle X-ray scattering diffraction and differential scanning calorimetry techniques using a heating rate of 2 degreesC/min after isothermal crystallization at 200 degreesC for 30 min. The copolymer containing random incorporation of 1,4-cyclohexylene dimethylene terephthalate monomer cannot be cocrystallized with the ethylene terephthalate moiety. Isothermally crystallized samples were found to possess primary and secondary crystals. The statistical distribution of the primary crystals was found to be broad compared to that of the secondary crystals. During heating, the following mechanisms were assumed to explain the multiple melting behavior. The first endotherm is related to the non-reversing melting of very thin and defective secondary crystals formed during the late stages of crystallization. The second endotherm is associated with the melting of secondary crystals and partial melting of less stable primary crystals. The third endotherm is associated with the melting of the remaining stable primary crystals and the recrystallized crystals. Due to their large statistical distribution, the primary crystals melt in a broad temperature range, which includes both second and third melting endotherms. The amounts of secondary, primary and recrystallized crystals, being molten in each endotherm, are different in various PET samples, depending on variables such as isothermal crystallization temperature, time, molecular weight and co-monomer content.