We have studied by means of X-ray scattering, DMA and densitometry the evolution of the semi-crystalline structure of poly(aryl-ether-ether-ketone) (PEEK) upon reheating specimens initially crystallized isothermally near T-g. Two regimes of reorganization behavior were evidenced. In the low-temperature interval (temperatures below T-g + similar to 50 degrees C), the slow dynamics of amorphous segments prevents large-scale rearrangements, strongly limiting the process of reorganization. Only lamellar-scale reorganization occurs, resulting in a slight increase of the crystal thickness and perfection, and in an increase of the constraints in amorphous regions, reflected by an increase of T-g. At higher temperatures, however, a larger-scale melting/recrystallization mechanism sets in. It consists in the recrystallization of whole lamellar stacks to give a state of overall lower free energy, with much thicker and dense crystals separated by larger and less constrained amorphous regions of lower T-g. (C) 2000 Elsevier Science Ltd. All rights reserved.