Mordenite membranes were reproducibly prepared by seeded hydrothermal synthesis onto commercial ceramic tubular supports. X-ray diffraction (XRD) patterns showed that mordenite was the only zeolitic material present in the membrane. The membranes were tested in the pervaporation of alcohol/water mixtures, giving good separation factors and a stable operation, even under acidic (pH = 3) conditions. Scanning electron microscopy (SEM) examination of the membrane revealed that a 40-70 mum thick layer of mordenite crystals filling the voids among the alpha-alumina support particles was likely responsible for the separation performance observed. The influence of the operating conditions on membrane performance was also studied. It was concluded that selective adsorption of water on the zeolite pores and small inter-crystalline defects controlled the separation mechanism in the mordenite membranes studied. (C) 2003 Elsevier Science B.V. All rights reserved.