This paper describes the preparation and properties of an inorganic membrane with permeability for H2 comparable to palladium and with over 99.9% selectivity over larger species like CO, CO2 and CH 4. The membrane is a composite formed by the deposition of a thin, 20 nm SiO2 layer on an alumina support. The alumina support is obtained by the deposition of a boehmite sol on top of a porous substrate, so as to create a uniform structure with small pore sizes. The permeation of the small gas species, H2, He, and Ne through the silica layer is analyzed in detail in order to obtain insight about the transport mechanism and the structure of the silica. The order of permeance through the silica layer is highly unusual, He > H2 > Ne, following neither molecular weight nor size. The order of permeation is quantitatively explained using a statistical mechanics approach, which takes into consideration the density of solubility sites for the various species and the vibrational frequency of the species within the sites. An extension of the Masaryk-Fulrath treatment for glasses combined with the Percus-Yevick model is used to estimate the vibrational frequency (7.0×1012 s-1), solubility site density (3.0×1026 m-3 for H2) and the average distance between sites (0.84 nm). This is the first time an inorganic membrane has been described in detail at the nanometer level.