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This paper describes a variant of the Rietveld refinement procedure applied to the characterization by X-ray scattering of disordered and small crystals as found in semicrystalline polymers. The program essentially focuses on the profile of the reflections and allows one to obtain the size, unit-cell parameters and lattice distortions (atomic displacements and paracrystallinity) of crystals in isotropic samples. The detailed influence of instrumental aberrations on the scattering is also considered. To illustrate the usefulness of the method, results obtained on PEEK [poly(ether-ether-ketone)] samples, cold-crystallized at various temperatures are presented. As the crystallization temperature increases, a closer packing of the chains in the 〈100〉 direction, a decrease in paracrystallinity along 〈100〉 and a thickening of the crystalline lamellae (〈001〉 direction) are observed. The lamellae are strongly microfragmented perpendicularly to the chain axis, resulting in coherently diffracting regions of sizes between 700 and 2000 nm3 only.