Surface plasmons (SP) or surface electromagnetic waves were excited at the silver-aqueous electrolyte interface with visible light using an attenuated total reflection arrangement. SP resonances were observed as a function of potential in perchlorate and halide electrolytes. At fixed wavelength, the resonance shifts to smaller wave vector as the electrode potential becomes more positive. Various contributions to this shift will be discussed: changes in electron density at the metal surface, ion adsorption and changes in the optical properties of the ionic double layer. Halide ion adsorption shifts the reflectivity: potential curve to more negative potentials. © 1980.