We have prepared and characterized organically-modified mesoporous silica membranes using supercritical CO2 fluid deposition. Supercritical CO2 fluid deposition has the potential to increase both diffusion and concentration of the reactants at the reaction site, a particular advantage when the reaction site is inside a small pore, where access can be limited. The unmodified mesoporous silica membranes were prepared by surfactant-templated synthesis on -alumina disk-shaped supports. These mesoporous silica supports had a mean pore diameter of 5nm. The alkyl modification of the silica membranes was achieved by the covalent attachment of octadecyldimethylchlorosilane (ODMCS) using triethylamine (TEA) as a catalyst in supercritical-CO2. Successful attachment was confirmed through infrared spectroscopic identification of the removal of the band due to isolated silanols accompanied by the appearance of bands due to methyl and methylene groups. We also directly measured a90% reduction in permeance of light gases through the modified membrane. The synthesis process did not degrade the support membrane, was relatively efficient, and was free of organic solvents required for traditional synthesis. The dominant transport mode in the membranes was Knudsen flow. Ideal separation factors for methane and propane over nitrogen were not increased relative to the bare support, indicating that neither the surface flow was enhanced nor the size selectivity improved for the membrane system studied. Supercritical CO2 fluid deposition was demonstrated as an effective technique for the reaction of the functionalized silanes on the surface of the silica membrane.