ZSM-5 zeolite membranes were prepared by in situ hydrothermal synthesis on macroporous or-alumina tubes coated with silicalite seeds. Seed particles were prepared by hydrothermal synthesis at 95 degrees C to mean size 0.4 or 2 mu m, purified of unconverted reactants, and suspended in a buffered aqueous solution. The support tubes were seeded by immersion in the particle suspension under ultrasonication. Membrane growth on seeded supports was carried out at temperatures 110-150 degrees C and different reaction times. An external polycrystalline layer gradually grows on top of the support, with siliceous deposits accumulating as deep as 100 mu m inside the pores of the support. Thermogravimetric analysis and nitrogen and argon adsorption measurements indicate graduate conversion of deposits from amorphous to crystalline. Pure gas permeation results are presented for membranes prepared using 0.4 and 2 mu m seeds under different compositions and temperatures.