Organosilane self-assembled monolayer (SAM) formation from solution on fused silica surfaces has been studied through the surface silanol group chemical reaction. The method is based on second-harmonic generation (SHG) efficiency as a function of pH. Direct experimental support is provided for the following organosilane SAM structure at the silica surface : (1) the organosilane SAM formation does not have any preference toward different surface sites; (2) during organosilane SAM formation process at the silica surface, most of the silica surface functional groups remain intact and therefore maintain their surface chemistry; (3) the silica surface is much rougher than the organosilane SAM on top of it and therefore provides more surface functional groups per unit area. This detailed understanding of the organosilane SAM formation may provide guidance for the proper application of SAM to various surface modification techniques, e.g., in microelectronic engineering or near-field optical chemical sensors.