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. IntroductionAmorphous silica 1 is an inexpensive and easily available commercial material that has many different industrial applications, including use as an adsorbent, a pigment, a catalyst support, a filler for rubber and other polymers, an electric insulator, an acoustic insulator, an optical material, and so on. It is also used for biological and pharmaceutical applications.Structurally, amorphous silica is a quite covalent oxide material 2 whose surface behavior is dominated by the chemistry of the terminal silanol groups 3 that are either isolated, vicinal, or geminal. 4 These groups, although being weakly Brønsted acidic, 5 are able to establish quite strong but easily reversible hydrogen bondings with polar molecules 6 that are the key phenomena occurring in adsorption-desorption cycles. Silanol groups lying nearby also make hydrogen bonds with each other, which results in chains of hydrogen-bonded hydroxyls 7 whose interaction with adsorbates is weaker than that of free silanols. 8 The silanols make the surface of hydroxylated silicas hydrophilic, and even more active in adsorption when wet. It is wellknown that hydrogen bondings also occur between the silica silanol groups and unpolar molecules such as hydrocarbons, 6,7,9 which allows silicas to be used for the adsorption of these compounds also. However, this interaction is weaker than that