An amorphous silica membrane with an excellent hydrogen/nitrogen (H-2/N-2) permselectivity of > 10 000 and a He/H-2 permselectivity of 11 was successfully synthesized on a gamma-alumina (gamma-Al2O3)-coated alpha-alumina (alpha-Al2O3) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H-2 permeance of > 1.0 x 10(-7) mol.(m(2).s.Pa)(-1) at >= 773 K. The dominant permeation mechanism for He and H-2 at 373-873 K was activated diffusion. On the other hand, that for CO2, Ar, and N-2 at 373-673 K was a viscous flow. At >= 673 K, that for CO2, Ar, and N-2 was activated diffusion. H-2 permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a gamma-Al2O3 mesoporous intermediate layer.