Understanding the effects of water vapor on gas permeation and separation properties of MFI zeolite membranes, especially at high temperatures, is important to the applications of these zeolite membranes for chemical reactions and separation involving water vapor. The effects of water vapor on H-2 and CO2 permeation and separation properties of ZSM-5 (Si/Al similar to 80) zeolite and aluminum-free silicalite membranes were studied by comparing permeation properties of H-2 and CO2 with the feed of equimolar H-2/CO2 binary and H-2/CO2/H2O ternary mixtures in 300-550 degrees C. For both membranes, the presence of water vapor lowers H-2 and CO2 permeance to the same extent, resulting in negligible effect on the H-2/CO2 separation factor. The suppression effect of water vapor on H-2 and CO2 permeation is larger for the less hydrophobic ZSM-5 zeolite membrane than for the hydrophobic silicalite membrane, and, for both membranes, is stronger at lower temperatures and higher water vapor partial pressures. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 153-162, 2012