The relative surface concentrations of octadecyltrichlorosilane (OTS) deposited on glass substrates in the presence of 11 different solvents were evaluated by X-ray photoelectron spectroscopy, with supporting evidence from contact angle measurements. The density of the observed OTS films cannot be explained entirely on the geometrical similarities or differences between the solvent and the octadecyl moiety, as it was observed that n-pentane produces an OTS film much sparser than that of hexadecane or toluene. A new mechanism involving extraction of surface moisture into the bulk solvent, followed by OTS hydrolysis and subsequent deposition, is proposed. Aromatic solvents (benzene and toluene) which are capable of extracting significant amounts of water from the substrate surface yield the densest OTS films, indicating a preference for OTS hydrolysis in the bulk solvent phase rather than at the substrate surface. A novel protocol was developed to quantitate the surface water extracting capacities of pentane and toluene. Dehydrated silica powder was rehydrated with D2O, followed by extraction with solvent, and the dissolved heavy water was trapped by treatment with phenyllithium to form deuterobenzene. The ratio of mass 79 (deuterobenzene) to mass 78 (benzene), determined by GC-MS, enables the quantitation of extracted D2O. The results show that toluene can extract significant amounts of D2O from the silica surface, whereas n-pentane is much less effective. OTS dissolved in anhydrous toluene only reacts with the surface silanol groups, producing a low density film. However, the same toluene, when spiked with a small quantity of D2O, forms an OTS film nearly twice as dense as that obtained under anhydrous conditions. The optimum quantity of moisture for the formation of a closely-packed monolayer is about 0.15 mg/100 mL of solvent.