We have studied the isothermal cold crystallization of poly(ether-ether-ketone) (PEEK) by atomic force microscopy, densitometry, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and small- and wide-angle X-ray scattering (SAXS and WAXS), A two-stage process was clearly evidenced. The first stage results in the progressive consumption of free amorphous regions by growing spherulites. Lamellae grow in tightly packed bundles, with no evidence for a random insertion mechanism. DSC and DMA consistently show the progressive disappearance of the alpha(1) relaxation typical of the initially amorphous PEEK. Simultaneously, a new relaxation appears a few degrees higher (alpha(2) relaxation) and develops at the expense of alpha(1) relaxation. It corresponds to the formation of amorphous regions constrained by neighboring crystals. On the basis of a simple simulation, we show that the experimental data in this second stage of crystallization can be accounted for by a simple stack thickening process in which new lamellae are progressively added in parallel to previously formed lamellar stacks.