Permeation of pure gases and binary gas mixtures through glassy polymeric membranes have been investigated. The separation factors of Langmuir permeation environment αH and Henry permeation environment αD

were introduced and their relationship with apparent separation tactor α was derived. The effects of feed pressure

composition

temperature and glassy transition of polymer on selective permeability were analysed and the applicabilities of dual-sorption dual-mobility model and its extended formula used for the description of permeation of gas mixtures were also verified experimentally. The phenomena occurring sometimes in practice that "the presence of a slow gas can reduce the permeability of a fast gas

the presence of a fast gas can increase the permeability of a slow gas" and that "the ratio of permeabilities of pure gases is usually larger than the practical apparent separation factor" were discussed with theoretical analysis and experimental data. It was found that these phenomena are not universal and may change conversely un-der specific conditions.