The intensity of second-harmonic light generated from plasmon surface polaritons at thin film silver electrodes in contact with an aqueous electrolyte exhibits a strong dependence on electrode potential for a variety of electrochemical systems. The surface plasmons enhance the electromagnetic field at the surface, thereby greatly increasing the second-harmonic intensity at the interface. We have measured the potential dependence of surface plasmon-enhanced second-harmonic generation at a silver electrode for aqueous solutions of sodium perchlorate, sodium thiocyanate, urea (a strong adsorbate), and for a buffered solution of lead acetate in the underpotential deposition region where a monolayer of lead is deposited onto the silver. The results can be interpreted by a simple theory relating the second-harmonic signal to the static electric field at the surface; to first order this field is proportional to the excess charge density on the metal. © 1984.