ZSM-5-type zeolite membranes were synthesized by hydrothermal reaction on the surface of a porous a-alumina support tube of 2.8 mm o.d. and 1.9 mm i.d. The average pore size of the tube was about 150 nm and the void fraction was 0.45. The silicon source was a fine silica powder and the template was a mixture of tetrapropylammoniumhydroxide and bromide. Three zones were observed in the film formed on the support tube. The top zone was a polycrystalline zeolite layer and the inner zone was a layer of cu-alumina macropores partially filled with deposits. The intermediate zone was a mixture of alumina particles and deposits. The permeance of the zeolite membranes increased with decreasing thickness of zeolite layer, but permselectivity was not connected with membrane morphology. The permselectivity of n-butane to i-butane was 10-50 in the range of 30-100 degrees C, which showed practical molecular sieving ability of the formed membranes.