Some samples of tetrafluoroethylene copolymers, containing perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, and hexafluoropropene, were studied by small-angle X-ray scattering in order to investigate the correlation among the crystalline lamellar thickness and both the comonomer content and the molecular weight. The lamellar thickness was determined from the identity period of the one-dimensional scattering function and from the analysis of the one-dimensional correlation function. The crystallinity and the volume of the crystalline unit cell were determined by wide-angle X-ray scattering. Some considerations are reported on the partial inclusion in the crystall cell of the -CF3 groups and on the expulsion into the amorphous regions of the -O(CF2)(2)CF3 groups.