A highly hydrogen permeable silica membrane, referred to as Nanosil, was obtained by chemical vapor deposition of a thin SiO2 layer on a porous Vycor glass support. This composite membrane showed good permeance (similar to10(-8) mol m(-2) s(-1) Pa-1) for the small gas molecules (He, Ne, and H-2) at 873 K with high selectivity (similar to10(4)) over other larger gas molecules (CO2, CO, and CH4). The characteristics of gas transport on the Vycor and Nanosil membrane were investigated with several gas diffusion models. The experimental gas permeation data on Vycor glass could be explained by the occurrence of Knudsen diffusion in parallel with surface diffusion. The permeance of the small gas molecules (He, Ne, and H-2) on the Nanosil membrane was activated, and increased as temperature increased. However, this permeance was limited at high temperature because of the limited pertneance on the Vycor support. The gas permeance on the deposited silica layer was obtained by applying a series analysis of gas permeation on the combined silica layer and Vycor support composite system. The order of permeance through the silica layer was He > H-2 > Ne which was the same as that through vitreous silica glass, but occurred with lower activation energies. The order of permeation of these small gas molecules did not follow either mass or molecular size but could be explained using a statistical gas permeance model.