ZSM-5 membranes were crystallized on tubular TiO2 supports and evaluated in the permeation of a 50%/50% 1-butene/i-butene mixture. If some of the water in the standard recipe 90 SiO2:0.225 Al2O3:1 Na2O:3.6 TPAOH:1.8 TPABr:1800 H2O was substituted by the same molar amount of short-chain length alcohol, the fluxes and permeances increased remarkably but the 1-butene/i-butene shape selectivity decreased only slightly. This experimental finding is attributed to additional non-zeolite micropores which are formed by the presence of alcohols in the synthesis batch. Since the formation of this additional pore system is linked to a decrease of the crystal size in the membrane layer, the increased length of grain boundaries could represent the structural origin of these additional non-zeolite pores. In a membrane preparation if 25% of the molar amount of water was substituted by ethanol, the 1-butene flux increased by the factor of two, but the mixture separation factor decreased by only 30%. Therefore, relatively thick ZSM-5 layers of about 30 mu m gave 1-butene permeances of the order of 1 m(3)(STP) m(-2) h(-1) bar(-1) with permselectivities of 20 and mixture separation factor of 6. The testing of the ZSM-5 membranes took place under practice-relevant conditions, i.e. with undiluted feeds and without applying sweep gases or reduced pressure on the permeate side. (C) 2010 Elsevier B.V. All rights reserved.