A new cobalt(II)-based metal-containing ionic liquid (MCIL), 1, has been prepared that reversibly and selectively absorbs O-2 with an associated colorimetric change. This MCIL was impregnated into a porous PTFE polymer support, and the resulting supported membranes show higher O-2 sorption over N-2 with increasing loading levels of 1. The O-2/N-2 selectivity of these 1-impregnated membranes was determined to be as high as 26 +/- 8 at 25 degrees C when similar to 0.9 bar of feed gas pressure was applied. The absorbed O-2 can be removed by moderate heating of the samples from 25 degrees C to 60 degrees C in vacuo. The high level of O-2 sorption in these 1-impregnated membranes was maintained for at least five successive sorption/desorption cycles.