Infrared spectroscopy was used to probe the reaction of organosilanes with a fumed silica immersed in supercritical fluid CO2 (SCF CO2). Venting of the CO2 solvent eliminates the experimental difficulties associated with solvent absorption of the infrared radiation and enables repeated surface treatment cycles without disturbance to the amount of silica in the beam. This stability is requisite for detecting infrared bands due to adsorbed species in the spectral region containing the strong metal oxide bulk modes. SCF CO2 has been shown to extract water from silica, and we now exploit this feature for the silane treatment of silica particles. This utility of CO2 as a solvent for the reaction of organosilanes with silica is demonstrated with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The HMDS reaction in SCF CO2 proceeds according to the conventional gas-phase process even though the ammonia generated as a byproduct reacts with the CO2 to produce ammonium carbamate. The volatile ammonium carbamate is weakly physisorbed on the surface hydroxyl groups and is easily removed with evacuation or by purging. Moreover, carbamate formation can be completely avoided by performing the reaction at relatively low CO2 pressures. Physisorption of OTS from SCF CO2 does occur via a weak interaction with the surface hydroxyl groups. Although a small amount of OTS is hydrolyzed by the residual water present in the SCF CO2 and adsorbs on the silica. the amount hydrolyzed is much lower than that found with the use of tradition nonaqueous solvents.