Electrocapillary equations are analyzed for partially polarizable interfaces between two immiscible electrolyte solutions (ITIES), and the determination of thermodynamic and free zero charge potentials at the oil/water interface is considered. The difference between potentials corresponding to the minimum of differential capacitance and maximum of electrocapillarity curves is caused by faradaic processes and nonideal polarizability of ITIES. The difference depends on the contribution of penetrating ions to the surface charge. Because the inner layer parameters are nonlinear functions of the potential difference at ITIES, the minimum capacitance potential shifts with increasing electrolyte concentrations even when there is no specific ion adsorption.