The crystallization kinetics and morphology of PEN/PET copolyesters were investigated using differential scanning calorimetry (DSC), time-resolved small-angle X-ray scattering (TR-SAXS), and transmission electron microscopy (TEM). The Avrami exponents obtained using DSC were approximately 3 for homo PEN and 4 for all the copolyesters. The 3-parameter Avrami model was successfully fitted to the invariants with respect to the time obtained from TR-SAXS, and the exponent values were similar to those obtained from DSC. Moreover, the Avrami rate constants obtained from TR-SAXS showed marked temperature-sensitive decreases in all samples, like those obtained from DSC. This indicates that not only could changes in morphological parameters be obtained from the analysis of TR-SAXS data but also crystallization kinetics. The changes in the morphological parameters determined from the SAXS data indicate that the minor components, dimethyl terephthalate (DMT) segments, are rejected into the amorphous phase during crystallization. In the TEM study, copolyesters crystallized at temperature above 240 degrees C grew into both the alpha- and beta-form, although 240 degrees C is the optimum condition for the beta-form crystal. (c) 2005 Wiley Periodicals, Inc.