Hydrogen permeation through a Pd8Y0.23Ru alloy membrane at 300 oC~450 oC has been investigated in the presence of CO2. The results show that the hydrogen permeability is greatly decreased by CO2 addition. The higher the CO2 concentration is, the more significant the reduction of hydrogen permeation will be. The coexisting CO2 gas has a poison effect on Pd8Y0.23Ru alloy membrane. The poison effect at 450 oC is stronger than at 300 oC. The reduction rate of hydrogen permeation decreases with the permeation duration. Besides poison effect, the negative effects of CO2 on H2 permeation are also caused by two other factors. Firstly, for higher concentration of CO2 (>3%), mass transfer of H2 ?is blocked by CO2 barrier layer near the membrane surface, which is the main reason for hydrogen permeation reduction. Secondly for lower concentrations (<1%), the main reason is the effects of CO2 adsorbing on the membrane and occupying active dissociation sites of hydrogen.