The pervaporation performance of mordenite membranes prepared by seeded hydrothermal synthesis was improved by a new post-synthetic treatment in which the membranes were hydrothermally treated at 180 degrees C under moderately alkaline conditions. The effect of the post-synthetic treatment was evidenced by pervaporation measurements, X-ray diffraction, scanning electron microscopy and chemical analysis of the solution after treatment. The post-synthetic treatment was found to be highly effective to improve the performance of mordenite membranes in the pervaporation of water/ethanol mixtures. The dissolution of Si-rich amorphous and zeolitic materials, possibly combined with further crystallization processes during post-synthetic treatment are invoked to explain the results obtained. (c) 2005 Published by Elsevier B.V.