Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane at 600-650 ℃ on a porous a-alumina tube with pore size of 110-180 nm or 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 membranes formed on γ-alumina coated α-alumina tube showed H2 permeances much higher than the SiO2 membranes formed on the a-alumina tube. This indicated that the g-alumina film was effective in improving the H2 permeance and H2/N2 selectivity. The permeation tests with CO2, N2, CH4, C3H8 and i-C4H10 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 composed of permeances due to the activated and Knudsen diffusion mechanisms. The contribution of Knudsen diffusion pores decreased to 0.02 when the γ-alumina film was modified at 650 ℃ until P(fe)=50 Pa.