The melting behavior of semicrystalline samples of PEEK crystallized from the glassy state was investigated by differential scanning calorimetry (DSC) and simultaneous time-resolved small-angle (SAXS) and wide-angle (WAXS) X-ray diffraction. The long spacing L and the invariant were computed from the Lorentz SAXS intensity curves. The amorphous L(a) and crystalline L(c) thickness as well as the linear degree of crystallinity upsilon(c)(lin) were obtained from the correlation function gamma(r). The wide-angle and small-angle integrated intensities were correlated to the degree of crystallinity obtained by DSC throughout the melting. The double-melting behavior of samples prepared by annealing at temperatures below 315 degrees C can be attributed to a melting-recrystallization process of a single population of crystals. The structural parameters of the reorganized lamellae in the different samples become identical as soon as the temperature exceeds the annealing temperature. Annealing at temperatures corresponding to the second melting point region (340 degrees C) generates a sample containing two populations of lamellae with a melting behavior very different from that of the samples prepared at lower temperatures.