The influence of mixing CO2 and H2O on the permeation of H-2 through the two different types of the Pd membranes was investigated. We used a thin metal membrane on an alumina support tube prepared by CVD and a pore-filling-type membrane that was constructed by embedding nano-sized Pd particles in the pores of an alumina support tube. The permeation rate at 573 K decreased with the increase of CO2 and H2O ratio. The decrease of the permeation rate of both gases on the pore-filling-type membrane was smaller than that on the CVD membrane. In particular, irreversible decrease of the flux appeared over the CVD membrane during mixing of CO2 at 573 K. That is, the permeation rate of H-2 decreased in response to mixing with CO2 and was not easily recovered by the subsequent supply of pure H-2 at the same temperature. This influence was due to the surface carbonous species produced by the reaction of CO2 and H-2 as observed by XPS and TPD analysis. It was not easily desorbed at 573 K and blocked the surface sites for the adsorption of H-2. At 673 K, the influence of CO2 was not observed, despite the formation of CO by the reverse water gas shift reaction because CO was easily desorbed from the membrane at this temperature. (c) 2012 Elsevier B.V. All rights reserved.