The contact electric resistance method (CER) has been applied for in situ investigation of the charge transfer through the semiconductor surface under potentiostatic control in electrolytes. Single crystals of n- and p-GaAs were investigated. As a result of changing the electrochemical potential, two simultaneous tendencies of surface resistance changes were observed. The first type of resistance change, which occurs immediately after application of the potential, is caused by band bending at the surface charge region. The second type of resistance change, characterized by a power law dependence on time, is caused by adsorption and / or oxidation processes at the surface. The surface resistance sharply increases close to the As/As2O3 (As/HAsO2) equilibrium potential, but does not change at potentials close to the Ga/Ga(OH)3 equilibrium potential. In addition, the influence of the sulphur content of the electrolyte on the surface resistance was investigated in 0.01M, 0.1M and 1M Na2S borate buffer solutions. At potentials lower than −0.8 VSCE sulphur was found to adsorb competitively on GaAs and to prevent adsorption of borate and hydroxyl anions, resulting in reduction of the surface resistance.