Zeolite ZSM-5 membranes were grown on porous alpha-A1(2)O(3) disks by in-situ hydrothermal synthesis at 175 degrees C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. Extensive experimentation was carried out to find compositions that gave continuous polycrystalline films. The films grown with the optimized composition were about 10 mu m thick and consisted of well-intergrown crystals of about 2 mu m size. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185 degrees C, respectively.