Facilitated transport of oxygen was investigated in ethyl cellulose membranes containing cobalt(II) meso-tetrakis (substituted phenyl) porphyrins [CoTPP, CoT(2-Cl)PP, CoT(4-Cl)PP, CoT(4-MeO)PP, and CoT(2,4-2MeO)PP] as fixed oxygen carriers. The oxygen permeability (P-O2) and oxygen/nitrogen selectivity (P-O2/P-N2,) Of the membranes containing oxygen carriers increase with a decrease in the upstream gas pressure, but the nitrogen permeability(P-N2) is almost independent of the upstream nitrogen pressure. This indicates that the fixed oxygen carriers in the polymer membranes can reversibly interact with oxygen and facilitate oxygen transport in the membranes. The study on the influences of the substituents in the cobalt(III) porphyrins and the fifth ligand (imidazole or pyridine) on the membrane permeation behaviors shows that the porphyrin complex with an electron-accepting substituent in the meso-phenyl ring or with imidazole as the fifth ligand could increase the permeability and oxygen/nitrogen selectivity of the membranes much more than that with an electron-donating substituent or with pyridine as the fifth ligand.