A mordenite membrane was prepared on the outer surface of a porous a-alumina tubular support by a secondary-growth method. Permeation tests for water/methanol/hydrogen mixtures were performed at 423 and 473 K. A mordenite membrane free from non-zeolitic pores allowed us selective permeation of steam and blocked the permeation of hydrogen and methanol. In order to elucidate the mechanism of such selective permeation behavior, adsorption, permeation, and structural properties of the mordenite membrane were discussed. As a result, it was considered that selective permeation of steam against hydrogen was based on its preferential adsorption of steam, while boundaries between mordenite crystals in the membrane may play a key role for inhibiting the permeation of methanol.