By using reflection Second-harmonic generation (SHG), the adsorption of triethylamine (TEA) at the silica/CH3CN interface has been studied with the help of two molecular probes, trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (DMP+I-) and trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium (DP). At the silica/CH3CN interface, DMP+ cations are adsorbed on SiO- sites by charge-charge interaction, while DP molecules are adsorbed on SiOH sites by forming hydrogen bonds. The saturation surface density is 3.10 x 10(13) cm(-2) for DMP+ cations and is 1.85 x 10(14) cm(-2) for DP molecules. The free energy of adsorption Delta G(ads) is -25.5 kJ mol(-1) for DMP+ cations and is -22.0 kJ mol(-1) for DP molecules. The addition of TEA into the solution causes dramatically different changes in the interfacial densities of the two probes : The surface density of DP molecules is decreased by a factor of 5 when the TEA concentration reaches 2 mM, while that of DMP+ cations is increased by a factor of 5 under the same condition. From the interfacial behavior of the two molecular probes that are coadsorbed with TEA and TEAH(+) at the silica/CH3CN interface, the free energy of adsorption and the base-ionization constant of TEA. at the silica/CH3CN interface have been extracted, Delta G(TEA) = -20.8 kJ mol(-1) and K-b = 0.0173.