Novel metal-containing ionic liquids (MCILs) with O-2 absorbability were synthesized by mixing a N, N'-bis(salicylidene) ethylenediamine cobalt(II) (Co(salen)) complex and ionic liquid-based ligands. The couple of anions of the ionic liquid-based ligands, trihexyl(tetradecyl) phosphonium N-methylglycinate and trihexyl(tetradecyl) phosphonium bis(trifluoromethanesulfonyl) imide, coordinated to Co(salen) complex and gave negative charge to the neutral Co(salen). The negatively chaged Co(salen) complex with the ionic liquid-based ligands and the bulky trihexyl(tetradecyl) phosphonium cation allowed to form Co(salen) complex-based MCILs. The MCILs could chemically and selectively absorb a large amount of O-2 and could function as an O-2 carrier in supported MCIL membranes. The O-2 permeability of the MCIL-based facilitated O-2 transport membranes were higher than those of conventional fixed-O-2 carrier membranes as well as polymer membranes. The fast O-2 transportation could be realized from the high diffusivity of the O-2 complex, which was formed via the reaction between MCIL and O-2, owing to the liquidity of the MCILs.