The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures T c = 100 fcir#circ;C and T c = 160 fcir#circ;C, i.e. in the middle of the relaxation region and close to its upper bound, respectively, are analyzed. During heating from room temperature, a structural rearrangement of the stacks is always found at T r ≃ T c + 20 fcir#circ;C. This process is characterized by a decrease of the linear crystallinity, irrespective of Tc; on the other hand, the WAXS crystallinity never increases with T below Tc+30fcir#circ;C. The lamellar thickness in the low-Tc sample decreases significantly after the structural transition, whereas in the high-Tc sample the lamellar thickness remains almost unchanged. In both, high …