The adsorption process of sodium-[4-(4＇-trifluormethylphenylazo)phenyl-1-oxyethylsulfonate] (NaTAPOS) at the water/1,2-dichlorethane interface was investigated by second harmonic generation and ellipsometry. The combination of both techniques yields the surface excess and the orientation of the amphiphiles within the adsorption layer. These data were used for an investigation of equilibrium properties of the system. The average tilt angle of the chromophore within the adsorption layer remains constant in the whole concentration range. The mean tilt angle was found to be 39 +/- 3 degrees and the area per molecule at maximum surface coverage is 45.2 +/- 6 Angstrom(2). The present study focuses on the impact of a base electrolyte on the adsorption process. Neither the limiting area per molecule nor the orientational order was changed by base electrolyte dissolved in the aqueous phase. The maximum surface coverage with and without base electrolyte is achieved at the same bulk concentration and is within the error limit identical; however, at lower concentrations a significantly higher surface excess is observed in the absence of base electrolyte. This is counterintuitive since one might expect an electrostatic shielding of the prevailing interaction. The adsorption isotherms with and in absence of base electrolyte can be described by Frumkin＇s equation of state. This model accounts for regular surface behavior and takes into account the lateral interaction of the absorbed species. The model fit yields two quantities: the free enthalpy of adsorption and the interaction parameter. Only slight changes of the free enthalpy of adsorption were observed by the presence of base electrolyte; however, a significant increase of the corresponding interaction parameter was observed. These findings are in contradiction to the Debye-Huckel theory and are explained by a model in analogy to the polyelectrolyte layer-by-layer adsorption.