N,N’-Dimethylformamide (DMF) and N,N’-dialicylidene ethylene diamine cobalt(II) (CoSalen) were added into the casting solution to improve both the gas permeability and oxygen/nitrogen selectivity of polycarbonate (PC) membranes. PC membranes formed by a simple dry method are known to have high oxygen/nitrogen selectivity but low gas permeability. A nonsolvent, DMF, was added into the casting solution to improve the membrane permeability, and an oxygen carrier, CoSalen, was added to improve the oxygen/nitrogen selectivity. The addition of DMF increased the membrane permeability but reduced the oxygen/nitrogen selectivity. On the contrary, the addition of CoSalen raised the selectivity but suppressed the permeability. To make a membrane of selectivity near the value of native PC membrane but with a higher gas permeation rate by adjusting the DMF and CoSalen contents in the casting solution, the highest oxygen permeation flux of CoSalen complexed porous PC membranes was 0.61 GPU(10(-6) x cm(3)/cm(2) . s . cmHg) at an O-2/N-2 selectivity of 4.7 at 35 degrees C. This membrane was obtained when 19 vol% of DMF and 4.8 wt% of CoSalen were added into the PC casting solution. This specific membrane, after a long period of storage, 400 days, had a higher oxygen flux (0.88 GPU) which was obtained without any loss in selectivity. At a lower temperature, 5 degrees C, an even higher oxygen selectivity of 7.9 was obtained.