The Xe-129-NMR method was used for evaluation of the sizes of free volume elements in amorphous glassy materials-random copolymers of tetrafluoroethylene and perfluorodioxoles of different structures. Xe-129 chemical shifts were measured at different pressures up to 1060 Torr. The zero pressure chemical shifts obtained by extrapolation were used for calculation of the diameters of microcavities having spherical and cylindrical shapes. The sizes of microcavities are consistent with those found by means of positron annihilation lifetimes and inverse gas chromatography methods. (C) 2003 Elsevier Science Ltd. All rights reserved.