Potential-step transients for the reversible transfer of fluorescent ions across the interface between two immiscible electrolyte solutions (ITIES) have been calculated theoretically by combining the Lambert-Beer law with the concentration profile in the organic phase. When so many fluorescent ions are present in the organic phase that the absorption of excitation light becomes significant, the half-wave potential of fluorescence-potential curves shifts in the negative direction and both the fluorescence-time and fluorescence-concentration curves show a tendency to saturate. At lower fluorescent ion concentrations, the fluorescence intensity is proportional to the total charge transferred across the interface. Experimental data corresponding to these two cases are presented for Eosin Y transfer across the 1,2-dichloroethanewater interface.