The adsorption of a nonlinear optical molecular probing cation, trans-4-[4-(dibutylamino)styryl]-1-methyl-pyridinium (DMP+), and its coadsorption with organic and inorganic cations at silica/CH3CN interfaces have been investigated using reflection second harmonic generation. Our results indicate that the site-binding model offers a better interpretation over the electrical double-layer model for the adsorption and coadsorption of cationic species at silica/CH3CN interfaces in the low electrolyte concentration region from 0.02 to 5.0 mM.