In this study, we report competitive adsorption-driven separation of a water/methanol mixture in Pd-deposited silica membranes, which is induced by introducing hydrogen carrier gas as a third competitor. After replacing helium carrier gas by hydrogen carrier gas, water vapor permeance showed a slight decrease, whereas methanol vapor permeance significantly decreased. The water/methanol separation factor remarkably increased from 1.7-16.5 to 6.8-58.2 in the feed water content of 5.8-83.0 wt.%. From single vapor permeation tests in the presence of carrier gas (hydrogen or helium), it was confirmed that those permeation behavior was derived from stronger effect of the competitive adsorption between hydrogen and methanol vapor than that between hydrogen and water vapor. That is, hydrogen carrier gas dominantly inhibits adsorption of methanol vapor on the membrane surface, and the partial pressure of methanol on the membrane surface decreases, which leads to a decrease in methanol permeance with reduced driving force. In addition, temperature programmed desorption (TPD) results of water and methanol from Pd/silica particles also demonstrated that hydrogen carrier gas suppresses methanol adsorption on Pd/silica surface more dominantly than water adsorption. (C) 2009 Elsevier Inc. All rights reserved.