Polyurethane (PU) membranes own high gas permeability but low selectivity. In the past, some attempts to increase the gas selectivity by modifying the polymer structure but the effort were unsuccessful. There were other attempts to improve the gas separation by the addition of high affinity salt into a membrane, It was reported that increasing the amount of oxygen carrier salt (cosalen) into a polycarbonate membrane did increase the selectivity of oxygen to nitrogen especially at low temperature [G. Galland, T.M. Lam, J. Appl. Polym. Sci. 50 (1993) 1041]. An increase in salt addition reduced the gas diffusivity but increased the diffusivity ratio of oxygen to nitrogen. In another study, it was also found that the addition of high oxygen-affinity salt into a polycarbonate membrane did not greatly increase the solubility ratio of oxygen to nitrogen but it significantly increased the diffusivity ratio of oxygen to nitrogen [Rouh-Chyu Ruaan, Shih-Hsiung, Chen, Juin-Yih Lai, J, Membr. Sci. 135 (1997) 9]. In this study, the effects of oxygen carrier salt namely, cosalen, in PU membrane on gas separation performance was examined. Gas diffusion and sorption properties were examined by sorption measurements and dual mode analyses. The dual mode analysis showed that the gas separation in PU membrane was dominated by gas diffusion rather than gas sorption. The selectivity of O-2/N-2 was 8.6 and the oxygen permeability was 1.1 barrer for the PU membrane with 5 wt.% cosalen at 5 degrees C. The key issue of improving the gas separation performance of a polyurethane membrane was to increase the diffusivity ratio but not the solubility ratio.