Environmental separations, such as the removal of volatile organic compounds from air or water, require a new paradigm for membrane materials design: preferential permeation of the larger, more soluble component. Nanocomposite materials, based on mesoporous inorganic substrates with organic moieties decorating the porous surfaces, are excellent candidates for this paradigm; the architecture affords a degree of simultaneous control over both chemistry and free volume that is difficult to achieve with purely polymeric or purely inorganic materials. We synthesized nanocomposite membranes by chemically and physically attaching small organic molecules to the surfaces of mesoporous (similar to 5 nm) alumina substrates and evaluated them on their performance in removing toluene from nitrogen. Octadecyltrichlorosilane (OTS), phenyltrichlorosilane (PTS), and a triazine-based hyperbranched molecule were employed as organic modifiers. Separation factors for toluene over nitrogen were calculated by conducting vapor-gas permeation experiments at different toluene feed compositions. The modified membranes were generally solubility-selective, but the OTS and hyperbranched triazine treatments yielded significantly higher separation factors as compared to PTS treated membranes. Our results indicate that both chemical specificity and molecular level packing play an important role in achieving solubility-based separations. Compared to purely polymeric membranes currently employed for such separations, our best nanocomposites exhibit similar separation factors but much faster permeation rates. (c) 2005 Elsevier B.V. All rights reserved.