In this paper, we present results of morphological studies during long time melt crystallization and subsequent melting in poly(aryl ether ether ketone) (PEEK). Morphological changes were monitored via small angle X-ray scattering (SAXS). SAXS data were analyzed via a combination of the correlation and interface distribution functions. Our analysis indicates the following : (I) The semicrystalline morphology is best described by a three-phase, dual lamellar stack model. Stacks of a finite number of lamellae and interlamellar amorphous layers are separated from each other by interstack regions of amorphous material (liquid pockets). (2) Secondary crystallization occurs via the formation of secondary lamellar stacks within the liquid pockets. Secondary lamellae are thinner than primary lamellae (70 degrees vs 120 Angstrom), and the amorphous layer thicknesses are about 47 Angstrom in both stacks. (3) The low endotherm observed during a heating scan is associated with the melting of the secondary lamellae. (4) At room temperature, the semicrystalline PEEK material is in a state of dilational stress (negative hydrostatic pressure) which may originate from the secondary crystallization process in constrained liquid pockets.