The effect of size and charge on the adsorption of organic and inorganic cations at the silica/CH3CN interface has been investigated using nonlinear optical molecular probes (NOMP). Dramatic size effect has been observed for the adsorption of organic cations at the silica/CH3CN interface. For organic cations with the structure of N(CnH2n+1)(4)(+), the free energy of adsorption is a linear function of the number of carbons in each chain (n), and it can be described by the following equation: Delta G(ad) = -30.60 + 3.07 n (kJ mol(-1)). The experiments carried out at different interfacial cation densities have demonstrated clearly that the steric effect dominates the interaction among coadsorbed cations in the interfacial density region from 2.8 x 10(13) cm(-2) to 1.96 x 10(14) cm(-2). Strong charge effect has been observed for the adsorption of divalent metal cations at the silica/CH3CN interface, which causes the free energy of adsorption to increase by about 8.2 kJ mol(-1)/charge.