The interaction of triethoxysilane (TES) and bis[3-triethoxysilylpropyl]tetrasulfane (TESPT) with silica has been studied by IR spectroscopy. TES has been adsorbed onto silica pretreated at different temperatures, from the gas phase and, also, by a liquid-phase procedure. The analysis of the Si-H stretching and deformation modes, the C-H stretching and deformation modes, the SiO-H stretching, and the C-O-Si stretching modes has allowed us to investigate the structure of the adsorbed species and propose a chemisorption/desorption mechanism. The spectral features of hydrogen-bonded TES and those of three different chemisorbed species (with the formation of one, two, and three Si-O-silica bonds, respectively) have been identified. It has been concluded that the first interaction involves terminal free silanols, which actually exist also on the wet silica surface. The second and third Si-O-silica bonds are established by opening the nearest siloxane bridges. The silanization process is faster when performed in the liquid phase, because of the presence of some water in the reaction medium and the long time of reaction applied. The presence of a small amount of water promotes the adsorption reaction, likely by hydrolyzing the adsorbate. The spectroscopic data suggest that TESPT is bonded, after typical liquid-phase contact, through two Si-O-silica siloxane bridges with one remaining ethoxy group. The extent of polycondensation of the silanes seems very limited, if at all, under our conditions.