When the electric potential is applied at the electrode to activate electrochemical reactions, part of the applied potential is dropped by the overpotentials and the rate of the reaction is determined by the potential difference at the interface between the electrode and the outer Helmholtz plane. Here we present a new method to measure the interfacial potential using the combination of the molecular probe and the electrochemical impedance spectroscopy. Using this method, we observed that the potential drop across the Helmholtz plane decreases logarithmically along with the concentration of the supporting electrolyte in the solution. As our method is available to any kind of electrochemical cells, it can be used to obtain information and control the properties of interfaces on the modified electrodes. (C) 2014 The Electrochemical Society. All rights reserved.