Zeolite NaA membranes have been synthesized by secondary growth on the external surface of a-alumina tubular supports using a semicontinuous system in which fresh gel was periodically supplied to the synthesis vessel. Compared to traditional batch methods, the procedure developed in this work provides a better control of the synthesis and crystallization conditions and is easier to implement at an industrial scale. The influence of the renewal rate during synthesis on the pervaporation and vapor permeation performance of the membranes was studied, with the best performance corresponding to the case where the gel was renewed every 20 min of synthesis time. The membranes obtained by the semi-continuous method displayed an excellent separation performance in the pervaporation of ethanol/water mixtures (e.g., a separation factor of 3600 at a water permeation flux of 3.8 kg/h m(2)). (C) 2004 Elsevier B.V. All rights reserved.