An electrochemical tunneling spectroscopy (ETS) system is constructed to study the electronic structure of semiconductor/electrolyte interfaces. By using this system, the tunneling current of GaAs (100) single crystal electrodes in solution was measured as a function of electrode potential at a constant sample-tip distance. Tunneling current flowed in the forward bias region; the current depended on the bias, i.e., the potential with respect to the flat band potential. It decreased by decreasing the bias and became zero near the flat band potential. This behavior can be explained by considering the potential dependence of the surface concentration of the majority carriers. The space charge layer within the semiconductor formed in the reverse bias region worked as an extra barrier for the tunneling process; therefore, no tunneling current was observed in this potential region. Current in the direction opposite to that in the forward bias region flowed as the reverse bias became very large, although this current was very small. In this potential region, the thickness of the space charge layer near the Fermi level of the tip became thinner and electrons were able to tunnel through the barrier. Effects of the doping density, the tip potential, the pre-set tunneling current and the initial sample potential as well as the surface treatment on the tunneling current-electrode potential relation were investigated in detail at D-GaAs(100) single crystal electrodes. The tunneling behaviors in the forward and reverse bias regions were discussed semi-quantitatively based on the calculation of surface electron concentration and the transmission coefficient through the space charge barrier, respectively.