In the modification of silica gel with aminoalkoxysilanes, silanols are present on the substrate surface and, after hydrolysis, in the silane molecule, The manner in which both types of silanols participate in the modification mechanism is elucidated, using nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic techniques. Deuterium exchange of surface hydroxyls before silanation is used in order to distinguish initial surface silanols from those created in the course of the modification, Surface hydroxyls are the active sites for the bonding of coating molecules. Small patches of unreacted hydroxyls remain after modification. An FTIR peak at 3731 cm(-1) is assigned to silane silanols, which are stable at low silane surface loadings.