Dense and thin pure silica CHA-type zeolite (Si-CHA) membranes have been synthesized on porous a-alumina supports and investigated for CO2 permeation and CO2/CH4 separation performance under dry and humidified conditions. Si-CHA membranes with ultra-high permeance were greatly affected by performance suppression due to concentration polarization under high pressure and low feed-rate conditions. Under high feed-rate conditions at 298 K and a feed pressure of 0.1 MPaG, the CO2/CH4 selectivity was 130 with a CO2 permeance of 4.0 x 10(-6) mol/(m(2) s Pa) for an equimolar CO2/CH4 dry gas mixture. The presence of water vapor significantly reduced CO2 permeance, but the performance improved as feed pressure increased. The CO2/CH4 selectivities at high pressure with and without water vapor were essentially the same.