Surface modification of mesoporous, inorganic substrates by organo-silanes to enhance the gas separation properties was studied with an emphasis on understanding the influence of the silane layer structure. Dimethyloctadecylchlorosilane (OCS) was used to create a polymer brush on the surface of mesoporous Vycor glass and octadecyltrichlorosilane (ODS) was used to create a polymerized silane layer. The membranes were characterized by gas permeation as well as indirectly by NMR spectroscopy on silica gel that had undergone same silane treatment as that of the membranes. All the modified membranes were selective for n-butane over nitrogen at 295 K. A variety of reaction and pretreatment conditions were studied to optimize the loading of monochlorosilanes. The effect of varying the silanization reaction temperature was minimal on the selectivity; however, the pure gas selectivity increased by a factor of three by varying the pretreatment conditions. For a typical OCS modified membrane, the pure n-butane/nitrogen selectivity was 7.8 as compared to the mixed gas selectivity of 72.9 for a 50% mixture of n-butane in nitrogen. This difference was probably due to the pore blocking effects of n-butane. In comparison to the OCS membrane, the ODS membrane was seven times more selective for n-butane over nitrogen in the pure gas experiments. However, in the mixed gas experiments, higher n-butane/nitrogen separation factors were observed for the membrane modified with OCS as compared to the ODS-modified membrane. (c) 2005 Elsevier B.V. All rights reserved.