To exceed the limitation of intrinsic separation performance derived from the pore structure of the skin layer of composite membranes, gamma-Al2O3/Pt-SiO2/stainless steel (SUS) membranes were prepared by the soaking-rolling method. The sweeping gas method as a permeation test was used under atmospheric pressure in both feed and permeate side. While the gamma-Al2O3/SiO2(100 nm)/SUS membranes fabricated for the purpose of comparison showed the Knudsen-dominated transport characteristics for the CO removal, permeation results of a H-2(99%)/CO(1%) mixture for the gamma-Al2O3/Pt-SiO2/SUS membranes presented a remarkable H-2/CO separation factor of 5.22-7.03 between 25 and 200degreesC due to the lower CO permeance. Considering surface diffusion induced by Pt catalysts placed under gamma-Al2O3 skin layer, the driving force of CO permeation is dominantly determined by the adsorbed phase of CO as well as the gas phase of CO in the lower side of skin layer. Therefore, a decrease in driving force of the CO permeation was induced by the CO confinement (poisoning) on Pt in the lower side of the skin layer, resulting in enhancement of CO removal efficiency, exceeding pure Knudsen system expected from the mesoporous gamma-Al2O3 layer. (C) 2004 Elsevier B.V. All rights reserved.