Permeabilities and selectivities of oxygen and nitrogen were studied using 3 flat polymeric mem-branes:silicone rubber, polyurethane, and cellulose acetate. Flow rate of permeates linearly increased with increasing pressure differences and was inversely proportional to membrane thickness. In silicone rubber membrane permeabi-lities of oxygen and nitrogen were 390 and 170 Barrers, respectively and selectivity of oxygen to nitrogen gas was 2.3. In polyurethane membrane permeabilities of oxygen and nitrogen were 1.5 and 0.39 Barrers, respectively and selecti-vity was 3.8. In addition the flow rate of permeates of the polyurethane membrane which was made using chloroform as solvent and vacuum drying was 10 times as large as those obtained from the polyurethane membranes made without using chloroform and vacuum drying. Only in freeze-dried cellulose acetate membranes the fluxes of oxygen and nitrogen were proportional to pressure differences. But the cellulose acetate membrane could not be used for oxy-gen/nitrogen separation process because the permeability of oxygen was almost the same as that of nitrogen.