A silica membrane having an excellent H-2/N-2 permeance ratio (over 800) was prepared by the counter-diffusion chemical vapor deposition method. The H-2/N-2 permeance ratio was kept for 82 h under 76 kPa of steam at 773 K. The H-2/H2O permeance ratio through the silica membrane was ca. 300 at 773 K. This is much higher than that through silica membranes prepared by a sol-gel method. The deposited silica layer was found in the gamma-alumina layer of the substrate by TEM observations, indicating that the silica deposition is controlled by counter-diffusion of the precursors. Steam stability tests for porous gamma-alumina substrates are also discussed. Pore size of the y-alumina layer increased with an increasing steam treatment period at 773 K. The pore size of they-alumina layer was ca. 9 nm after 52 h of steam treatment.