20 wt% RuCl3 . 3H(2)O-containing polycarbonate membranes are prepared by using co-solvent and using these metal-containing membranes, over 99% of pure oxygen (O-2) gas was recovered from a 1:1 mixture of O-2 and CO gas mixture at room temperature. Under the described conditions, the Ru complexes are inert to oxygen, but quite reactive to carbon monoxide gas molecules. Very stable Ru-carbonyl species are found to be formed during the gas permeation in the membrane, indicating that there are relatively strong interaction between CO gas molecules and the Ru complexes incorporated into the polycarbonate membrane. Such preferential interactions are believed to give rise to a drastic change in permeation selectivity of a organic polymer membrane.