The behavior of octadecyltrimethoxysilane (C18TMS) at the air-water interface for different subphases pH has been studied through surface pressure-area and surface potential-area isotherms combined with Brewster angle microscopy. C18TMS monomolecular behavior has been assigned to the one observed at pH around 8.0. Hydrolysis or hydrolysis followed by condensation occurs when pH is lowered below 5.0. Hydrolysis followed by condensation is also observed when the pH is raised above 10.5. The type of catalysis predominantly influences the product of the reaction. Condensation is performed at low pH under two constraints : steric hindrance between the C-18 tail chains and electrostatic repulsion between polar groups, resulting in a very low or nil polymerization degree of the condensation products around the isoelectric point (pH 3.8). Higher concentration of chloride ions gradually inhibits the second constraint, resulting in the formation of a linear polymer. Only steric hindrance has been observed under basic catalysis, producing a monodimensional polymer similar to the one obtained at low pH.