Hybrid organic-inorganic H-2-selective membranes consisting of single-layer or dual-layers of silica incorporating aromatic groups are deposited on a porous alumina support by chemical vapor deposition (CVD) in an inert atmosphere at high temperature. The single-layer silica membranes, which are made by the simultaneous decomposition of phenyltriethoxysilane (PTES) and tetraethylorthosilicate (TEOS), have good hydrothermal stability at high temperature and a high permeance for hydrogen in the order of 10(-7) mol m(-2) s(-1) Pa-1 at 873 K, while preventing the passage of other larger molecular gases such as CH4 and CO2. The dual-layer silica membranes, which are obtained from the sequential decomposition of PTES and TEOS, exhibit an extremely high permeance for hydrogen of 3.6 x 10(-6) mol m(-2) s(-1) Pa-1 at 873 K with a permselectivity of hydrogen over methane of 30. A normalized Knudsen based permeance method is applied to measure the pore size of PTES-derived silica membrane on the dual-layer silica membrane before treatment with TEOS. The method indicates that the pore size of the silica network is approximately in the range of 0.50-0.85 nm, which is higher than the characteristic length of pure silica membranes of 0.3 nm, accounting for the high permeance of the hybrid membranes.