Zeolite NaA membranes have been reproducibly prepared by seeded hydrothermal synthesis on the internal surface of porous alpha-alumina tubular supports. A cross-flow filtration technique has been developed to allow a controlled seeding of zeolite NaA crystals from a suspension according to transmembrane pressure, pH, seed suspension flow rate, crystal size and concentration. The optimal seeding weight gain was found to be around 0.40 mg cm(-2) of membrane area. With this procedure, zeolite membranes with selectivities up to 600 at fluxes of 0.50 kg m(-2) h(-1) were obtained in the pervaporation of 92:8 wt.% ethanol/water mixtures at 323 K. (c) 2005 Published by Elsevier B.V.