Zeolite ZSM-5 membranes were prepared by in situ crystallization on porous alpha-Al2O3 disks that contained a diffusion barrier to limit excessive penetration of siliceous species into the alumina pores. The barrier was introduced into the alumina pores by impregnating the porous disk with a 1 : 1 molar mixture of furfuryl alcohol (FA) and tetraethylorthosilicate (TEOS), polymerizing the mixture retained in the disk, and carbonizing the resulting polymer at 600 degrees C in N-2. Following carbonization, a partial carbon bum-off was carried out by catalyzed oxidation in 2% O-2-N-2 at 600 degrees C to generate a carbon-free region near the surface of the support. After zeolite crystallization, the remaining carbon and the organic structure directing agent were removed by calcination in air, at 500 degrees C. It was found that pure carbon does not support zeolite growth while the solid obtained from a mixture of FA and TEOS does, due to the presence of dispersed silica. Membranes synthesized using barriers have n-butane flux and n-butane/isobutane selectivity 2.7 x 10(-3) mol m(-2) s(-1) and 45, respectively, at 185 degrees C, which are, respectively, ea. 1.6 and 4 times as large as those of membranes prepared without the use of barriers. Electron probe microanalysis (EPMA) and X-ray diffraction (XRD) revealed that the internal layer of the barrier-pretreated membrane has smaller thickness and higher crystallinity accounting for the increased flux and selectivity.