Alumina-zeolite composite membranes were prepared on alumina porous tubular supports by single-step in situ hydrothermal synthesis. Hydrogen/n-butane separation factor was used as a criterion to evaluate the presence of defects. Five membranes of different qualities were selected. The hydrogen permeance through the membranes, plugged beforehand with two strongly adsorbing gases (water and n-butane), was followed with time at room temperature. The dynamic desorption thus observed was correlated to the quality of the membrane. (C) 2003 Elsevier B.V. All rights reserved.