Random copolymers of tetrafluoroethylene with minor amounts of fluorinated comonomers (hexafluoropropene, perfluoromethylvinylether and perfluoropropylvinylether) of different compositions are characterized mainly by X-ray diffraction but also by differential scanning calorimetry and density measurements in an attempt to establish the possible inclusion of the respective side groups into the crystallites. Structural characterizations are conducted at room temperature (where all the considered copolymer samples are in the less ordered crystalline form I) as well as at -40 degrees C (where all the considered copolymer samples are in the more ordered crystalline form II). Increases of the lateral dimension of the unit cell and decreases of the size and amount of crystalline domains, with comonomeric unit content, are observed which are similar for different comonomer units (of largely different bulkiness) as well as for the two crystalline forms. The reported experimental results suggest a substantial exclusion from the crystallites not only of the bulkier comonomeric units (from perfluoropropylvinylether), but also, at least for high comonomer compositions, of the smaller comonomeric units (from perfluoromethylvinylether and hexafluoropropene).