The evolution of structural parameters characterizing the semicrystalline morphology of poly(aryl-ether-ether-ketone) (PEEK), and its blends with poly(aryl-ether-imide) (PEI), has been determined as a function of temperature by means of small-angle (SAXS) and wide-angle (WAXS) X-ray scattering studies, using synchrotron radiation. By comparing the behavior of initially amorphous samples continuously heated in the beam and samples heated and cooled below and above their annealing temperature, a complete picture is drawn of the morphological changes occurring during the cold crystallization and subsequent heating-cooling of PEEK. Upon crystallization of an amorphous sample, new lamellae are progressively inserted in the free space left between existing ones. No decrease of the lamellar thickness is evident during this insertion mechanism. When the polymer is cooled below its last annealing temperature T-c, only reversible morphological changes occur, which can be explained quantitatively by the effects of thermal expansion, provided one takes into account the existence of strains in the crystalline lamellae created by their coupling to the noncrystalline interlamellar regions. When the polymer is heated above T-c, "irreversible" changes occur in the average thickness of the lamellar crystals and of the interlamellar noncrystalline regions, in the perfection of the crystals, and in the crystalline density. Cold-crystallized samples are shown to comprise a single lamellar population which undergoes a rapid melting-recrystallization process above T-c. The double melting behavior of cold-crystallized PEEK results from this melting-recrystallization mechanism.