Zeolite NaA membranes have been reproducibly prepared by seeded hydrothermal synthesis onto the inner-side of porous a-alumina tubular supports using a semi-continuous system in which the gel inside the tubes was periodically renewed. A cross-flow filtration technique was used to allow a controlled seeding of zeolite NaA crystals from a suspension. The influence of the renewal rate during the synthesis, as well as the seeding weight gain (SWG), on the pervaporation performance of the membranes prepared was investigated. An optimal SWG value was found to be around 0.15 mg cm(-2) of membrane area. The use of a semi-continuous system allowed to obtain zeolite membranes with selectivities up to 16,000 at fluxes of 0.50 kg m(-2) h(-1) at 323 K for the pervaporation of 90: 10 (wt.%) ethanol/water mixtures. (c) 2005 Elsevier B.V. All rights reserved.