Silylation reactions using liquid and supercritical. CO2 as the solvent were performed on both single surface (silicon wafers) and nanoporous (silica gel) silica samples. The alkylsilyl monolayers formed on single surfaces were characterized by wettability, ellipsometry, and X-ray photoelectron spectroscopy. Modified nanoporous silica samples were analyzed by chemical analysis. A range of different silanes including monochloro-, dichloro-, trichloro-, and dimethylamino-silanes was examined. The results indicate that dense carbon dioxide is a good solvent for silylation reactions, comparable or better than most solvents. Compared with the optimum conditions for silylation at solid-liquid interfaces, reactions in carbon dioxide are faster, although maximum bonding densities are slightly lower.