The outer surface of an α-alumina porous tube of 1.9-mm i.d. and 2.5-mm o.d. was coated with boehmite sol, which was calcined into a γ-alumina layer of 1.5-2.5-μm thickness. Then pores of the γ-alumina layer (average pore size ≃ 7 nm) were filled with SiO 2 formed by thermally activated chemical vapor deposition (CVD) of tetraethyl orthosilicate at 400-600°C. The reactant was continuously evacuated through the porous support tube by keeping the pressure inside the support tube at 1.7-1.8 kPa initially. At the end of the modification, the pressure was reduced to 10 Pa. This procedure was effective in occluding pinholes that might be formed by other methods. A SiO 2 film of 50-100-nm thick was also placed on the top of the porous γ-alumina layer. The hydrogen permeability of the composite membrane was 10 -7 -10 -8 mol.m 2 /s.Pa at 600°C, and the selectivity of H 2 to N 2 was higher than 1000. The membranes were stable in a steam atmosphere at 500°C