The adsorption behavior of the cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) on the Au(111) electrode surface was characterized using cyclic voltammetry, differential capacity, and chronocoulometry. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DeTATf has a multistate character. At low bulk DeTATf concentrations, the adsorption state is consistent with the formation of an adsorbed film of nearly flat molecules. At higher concentrations this film may represent a three-dimensional aggregated state. At negative potentials and charge densities close to 0 mu C cm(-2), the data suggest the formation of a film of tilted molecules oriented with the hydrocarbon tail toward the metal surface and the polar head toward the solution. A surprising result of this study is that DeTATf displays adsorption characteristics of a zwitterionic rather than a cationic surfactant. This behavior indicates that the adsorbed species is an ion pair.