In-situ Fourier transform infrared spectroscopy measurements were obtained by using an innovative equipment to study the heterogeneous reaction between a hydrolyzed porous methylsilsesquioxane film and hexamethyldisilazane (HMDS) dissolved in CO2 at supercritical conditions. Gas and solid infrared signatures were separated to obtain kinetic information of the heterogeneous reaction. A two-step reaction mechanism was observed: a fast first step controlled by kinetics and a second step controlled by the diffusion of the HMDS inside the porous material. Infrared information was used to derive a rate law expression of the silylation reaction between HMDS and Si-OH. A first order of reaction relative to the concentration of hydrophilic sites was observed with activation energy of 51.85 +/- 1.25kJ/mol.