Some quenched and annealed samples of tetrafluoroethylene copolymers containing perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, and hexafluoropropene were studied by small-angle X-ray scattering and differential scanning calorimetry. Small-angle X-ray scattering experimental spectra were analyzed by a fitting method with the profiles calculated from some theoretical distribution models of the lamellar thickness. The mathematical evaluation of small-angle X-ray scattering patterns has provided crystallinity values which were compared with those obtained by wide-angle X-ray scattering in order to obtain information on the organization of the lamellar stacks. The experimental results allow one to conclude that, during the annealing of the quenched fluorinated copolymers, at increasing temperatures, the melting of the thin lamellae takes place together with the appearance and the growth of nonlamellar crystallites.