Degradation mechanism of H-2 permselectivity under atmosphere of steam/N-2 = 3 at 773 K was investigated among amorphous silica membranes on gamma-Al2O3-coated alpha-Al2O3 or Ni-doped gamma-Al2O3-coated alpha-Al2O3 synthesized by counter diffusion chemical vapor deposition. Helium and H-2 permeance drastically decreased during the first few hours of hydrothermal exposure. On the other hand, N-2 permeance fluctuated during hydrothermal treatment in silica membrane on gamma-Al2O3-coated alpha-Al2O3. The degree of N-2 permeance change depended on the hydrothermal stability of the intermediate layer. Hydrogen permselectivity was affected by both the densification of amorphous silica and the sintering of the intermediate layer during hydrothermal exposure.