In this paper we discuss the effect of a supporting electrolyte on mirage experiments in electrochemical cells and we report data showing the dependence of the mirage signal on the concentration of the supporting electrolyte. Concentration profiles for all species present in the electrolyte can be calculated exactly under specific requirements. The mirage signal is then calculated as a function of the ratio of the concentrations of the supporting and the electroactive electrolytes, and the results are compared with the experimental data. This model allows calculation of the optical parameters of the supporting electrolyte and not just those of the electroactive species.