A new method to obtain silica aerogel particles using acetic acid as the condensation agent for silicon alkoxides in supercritical carbon dioxide (scCO(2)) is proposed. The objective of this study was to determine the mechanism of silica aerogel formation in scCO(2) using in situ analysis techniques. The synthesis and formation of silica aerogel particles was carried out by a modified sol-gel route, based on the hydroxylation and condensation of silicon alkoxides in scCO(2); both submicron and micron-size aerogel spheres were obtained. By means of in situ Fourier Transform infrared spectroscopy (FTIR), the activity of acetic, formic, benzoic, and chloroacetic acids were studied for the condensation of tetraethyl orthosilicate (TEOS). Formic and acetic acid gave slower rates than benzoic and chloroacetic acids. Increasing the concentration of acid and addition of extra water led to an acceleration of the reaction. The reactions were also studied as a function of temperature and pressure. Higher rates of reaction were obtained at higher temperatures and lower pressures. Results from particle formation studies indicated that by slowing the rate of reaction, precipitation and agglomeration of particles could be minimized. A submicron particle size range was obtained by depressurization of the sol-gel solution inside the reaction vessel, while the rapid expansion of supercritical solutions (RESS) process was found to yield particles in the size range of approximately 100 nm.