Charge transfer through thin layers of water between an STM tip and an Au surface has been investigated by using a scanning tunneling microscopy (STM) technique at a relative humidity of 95%. The plateau currents were found to appear at separations larger than several angstrom where the current decay stops. The polarity dependence of the electrochemical currents was not observed with STM junction between an Au tip and an Au surface, which was observed with STM junction between a Pt/Ir tip and an Au surface. The plateau currents at H2O saturation were measured to be 10-20% higher than those at D2O saturation at bias voltages above vertical bar 0.4 vertical bar V. The results were rationalized in terms of the electrochemical currents originating mostly from, the processes of OH- (OD-) or H2O (1320) on the positive surface with different conductivity between H2O and D2O, (C) 2005 Elsevier Ltd. All rights reserved.