Hydrolysis and condensation of spread monolayers of octadecyltrimethoxysilane (ODTMS) have been studied as a function of pH by means of in-situ epifluorescense microscopy, surface potential and compression isotherms within the pH range 0.5-13.5. The point of zero charge of the monolayer was found to be 3. At pH < 3 the ODTMS monolayer was positively charged whereas it was negatively charged at pH, 12. The hydrolysis was both acid and base catalyzed. Unlike what has been observed for corresponding three-dimensional sol-gel reactions, condensation occurred immediately after the methoxy groups had hydrolyzed, as evidenced by fluorescence microscopy. Large continous aggregates of condensed ODTMS were observed if the monolayer was allowed to undergo surface pressure relaxation at constant area. The time required before gelation was observed varied between a couple of minutes at extreme pH values to 10 h at intermediate pH. Depending on the pH, floc-like or gel-like structures were formed.