Time-resolved small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) analyses were used to study the isothermal crystallization and remelting of poly(ethylene isophthalate-co-terephthalate)s containing 0−10 mol % isophthalate unit. For each of the polymers considered, evidence of the formation of both primary and secondary crystals was found in the SAXS pattern and its invariant Q, as well as in the DSC thermogram, which showed multiple melting endotherms. The melting of secondary crystals was found to shift significantly toward the high-temperature region with increasing crystallization time, indicating that the secondary crystals become thick and perfect over time. The thickness of the lamellar crystals was found to be very sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization run, regardless of composition. The formation of secondary crystals, which favorably occurs during the later stages of crystallization, was foun...