The nature of secondary crystallization in poly(ethylene terephthalate) (PET) was examined during isothermal crystallization and subsequent melting by time-resolved synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) techniques. In one experiment, the process of isothermal crystallization was sustained over 72 h to induce a relatively large crystallinity (46%, by weight). The purpose of this experiment was to resolve the issue of controversial assignment for the crystal lamellar thickness (I-c) by the correlation function analysis of the SAXS data. Results suggest that a two-stage decrease mechanism exists in both long period (L) and I(c )during isothermal crystallization: (1) a significant decrease in the initial stage (primary crystallization dominant), and (2) a much slower decrease in the later stage (secondary crystallization dominant) that is nearly linear with log time. We attribute this behavior to the formation of thinner separate stacks of lamellae between the primary stacks by secondary crystallization. Both secondary and primary stacks can undergo a great deal of crystal perfection and rearrangement with time. From DSC measurements, a triple-melting behavior was observed in the samples crystallized at 205 and 215 degrees C for 1 h, and a double-melting behavior at higher temperatures of 225 and 231 degrees C for 2 h. Temperature scanning SAXS and MDSC directly characterize aspects of crystal perfection and melting. Consistent with some of the literature, we confirm that for short annealing ( similar to hour) at 200-220 degrees C, the first (low) endotherm is related to melting of secondary crystals, the middle endotherm is due to melting of primary crystals, and the third endotherm is due to melting of crystals reorganized during heating. With prolonged crystallization at 231 degrees C for 24 and 72 h, a single higher melting endotherm was observed even though SAXS experiments indicate a slight decrease in average lamellar thickness. In PET, eater exchange reactions contribute to unusual high mobility, allowing chains to avoid topological constraints such as entanglements and tie chains. The results suggest that the change in population of tie molecules in the non-crystalline phase reduces the entropy of melting causing an increase in T-m,, and that this overwhelms the contribution of the decrease in I,.