A highly hydrogen permselective composite silica membrane was obtained by depositing a thin silica layer on a porous alumina support by the chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) at 873 K in inert gas at atmospheric pressure. The silica/alumina membrane showed a high hydrogen permeance (similar to10(-7) mol m(-2) s(-1) Pa-1) with selectivity over CH4, CO, and CO2 in excess of 1000 at 873 K. Cross-sectional and surface images of the membranes obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the silica film deposited on the gamma-Al2O3 support was uniform with a thickness of 20-30 nm. This indicated that the high temperature thermal decomposition of tetraethylorthosilicate (TEOS) used in this work was excellent in controlling the uniformity and thickness of the silica film formed on the porous alumina support. On the fresh alumina support the permeance of gases (He, H-2, CH4, CO, and CO2) decreased with temperature and molecular weight in agreement with a Knudsen transport mechanism. However, on the silica/alumina membrane the permeation of H-2 and He was activated and increased with temperature. The transport mechanism for the small gas molecules (H-2 and He) through the silica membrane was analyzed using a permeation mechanism which involves the jumping of the diffusing molecules between adjacent solubility sites. The model analysis indicated that the structure of the silica layer is more open than that of vitreous silica glass, with larger interconnecting passageways and low activation energies for permeation, allowing for easier gas diffusion. (C) 2003 Elsevier B.V. All rights reserved.