Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane(TEOS) at 600 ℃ on a γ-alumina coated α-alumina tube with pore size of 6-8 nm. The forced cross flow through the porous wall of the support was very effective in plugging macropores. The silica modified layer was extended to a depth of 200 nm and the top surface of the film was quite smooth without pinhole. The silica membrane showed a hydrogen permeance of 1.41 X 10(-7) mol . m(-2).s(-1).Pa-1 and a H2/N2 selectivity of about 20 at a permeation temperature of 450 ℃. The permeation tests with CO2, N2, CH4 and C3H8 showed that a very small number of mesopores remained unplugged by the CVD. Permeation of hydrogen was explained by activated diffusion, and that of the other gases by Knudsen diffusion through the unplugged pores. Thus, the total permeance was comprised of permeances due to the activated and Knudsen diffusion mechanisms. The CVD-modified silica membrane was stable in a H-2-N-2 mixture of 450 ℃ for 100 h.