A silica membrane was produced by chemical vapor deposition using tetraethoxysilane (TEOS), phenyltriethoxysilane (PTES) or diphenyldiethoxysilane (DPDES) as the Si source. Amorphous silica was deposited in the mesopores of a gamma-alumina film coated on a porous alpha-alumina tube, by evacuating the reactant through the porous wall. Hydrogen permeance at a permeation temperature of 600 degrees C was of the order of 10(-7) mol m(-2) s(-1) Pa-1, and was not greatly dependent on the Si sources. The silica membrane produced using TEOS contained micropores permeable to both helium and hydrogen, but CO2 and larger molecules were only slightly permeated through those mesopores which were left unplugged. The silica membrane produced from DPDES showed a single-component CO2 permeance equivalent to that of single-component He, and CO2/N-2 selectivity was approximately 9 at a permeation temperature of 30 degrees C. When a mixture of CO2 and N-2 was fed, however, CO2 permeance decreased to the level of N-2 permeance. The H-2/N-2 selectivity, determined from single-component permeances to H-2 and N-2, was approximately 100, and these permeances remained unchanged when an equimolar mixture of H-2 and N-2 was fed. Thus, the DPDES-derived membrane possessed two types of micropores, abundant pores through which helium and hydrogen permeated and a small number of pores in which molecules of CO2 and N-2 were permeable but not able to pass one another. Neither meso or macropores remained in the DPDES membrane.