(gamma-Aminopropyl)triethoxysilane (APTES) and simpler molecules having at least one common function with APTES (i.e., propylamine and mono-, di-, or triethoxyalkylsilane) were adsorbed at the silica/heptane interface at 25 degrees C. The hydrolysis of ethoxy functions by surface water is observed only for APTES, which stresses the catalytic effect of the amine function. When the water surface content of silica is decreased by a heat treatment, a decrease of both APTES surface concentration and adsorption enthalpy is observed, whereas they increase in the case of propylamine. This behavior is close to that observed in the case of triethylsilanol adsorption on silica which is interpreted as an enhancement of siloxane bond formation by surface water.