Free volume in a big group of glassy polymers distinguished by a wide range of gas permeability (P in the range 1-10(4) Barrer) was studied using the positron annihilation lifetime (PAL) method. To take into account an additional channel of decay of positronium (Ps), caused by the reaction with sorbed O-2, measurements were performed both under ambient conditions and under a nitrogen atmosphere. The finite-term and continuous lifetime analyses (PATFIT and CONTIN programs) were used for the treatment of the positron annihilation decay curves. It was shown that better statistical fitting parameters can be obtained for four component PAL spectra of a vast majority of the glassy polymers studied. The presence of two Ps lifetimes (tau(3) = 1.7-3.3 ns and tau(4) = 2.7-10.9 ns) indicates that size distribution of free volume elements (FVE) in glassy polymers is typically bimodal. The presence of smaller and larger FVE (R-3 = 2.5-3.5 Angstrom, R-4 = 3.5-7.0 Angstrom) was proved by PATFIT analysis of the data. Continuous analysis clearly indicated two broad peaks, especially for more permeable polymers. As permeability and free volume of the polymers decrease, the two peaks start to approach and overlap each other, ending with one broader or asymmetric peak for the less permeable polymers. In the presence of oxygen both lifetimes tau(3) and tau(4) are shifted to shorter values; this shift is more pronounced for the polymers with larger radii R-4 Of FVE. A novel correlation of diffusion selectivity of glassy polymers with the size of FVE is demonstrated.