A theory has been proposed for optical beam deflection at the electrodeelectrolyte boundary due to the interfacial transfer of an ionic component of a solution, containing excess of a single-charged background electrolyte, induced by an abrupt change in the electrode polarization. It has been demonstrated that the optical signal is generated not only by surface-transferred ionic species but also by a redistribution of background ions. As a result, it depends on the diffusion coefficients of all ionic components as well as on the derivatives of the solution refraction ind" with respect to the concentrations of both surface-transferred and background ions. The previous simplified approach disregarding the optical effect due to background electrolyte redistribution may in some cases lead even to erroneous conclusions on the direction of the interfacial flux or on the type of the ion transferred across the interface. Conditions for appropriate choice of the background electrolyte are formulated to avoid these complications.