The potential-induced accumulation of electrolyte ions at the water/1,2-dichloroethane (DCE) electrochemical interface has been examined by total internal reflection second harmonic generation (TTR SHG). TLR SHG experiments provide the first direct optical measurements of the accumulation of the base electrolytes, tetraphenylarsonium (Ph(4)As(+)) and tetraphenylborate (Ph(4)B(-)), in the interfacial region between two immiscible electrolyte solutions (ITIES). The standard Galvani potential for ion transfer across the water/DCE interface for the tetrabutylammonium (TBA(+)) is measured from the SHG data. Using the Ph(4)As(+) ion as a reference, a value of 219 +/- 5 mV is determined for TBA(+). In addition, optically determined potential-dependent concentration profiles for the Ph(4)As(+) and Ph(4)B(-) ions are correlated with the modified Verwey-Niessen model for surface excess. Results suggest that less than 10% of the potential drop across the water/DCE interface occurs in the organic phase.