The protonation, hydrolysis, and condensation kinetics of octadecyldimethylmethoxysilane (OMMS) and octadecyltrimethoxysilane (OTMS) at the air/water interface were investigated using a monolayer trough. OTMS chemical condensation within physically condensed phases was observed in transferred monolayers using fluorescence microscopy. Molecular area increases and decreases attributed to protonation and hydrolysis, respectively, of silane methoxy groups were measured by a surface balance. These area changes at constant surface pressure suggested a stepwise protonation and hydrolysis of the three OTMS methoxy groups. In contrast, only a single protonation and hydrolysis event was observed for monofunctional OMMS. The influences of monolayer spreading time, silane packing density, and subphase pH an the reaction kinetics are presented.