The electrochemical oxidation and reduction of the Cu1+/Cu2+ redox system at p-GaAs electrodes have been investigated by impedance spectroscopy. This reaction is a diffusion-controlled process via the valence band, which competes well with the anodic decomposition reaction. The method of impedance spectroscopy makes it possible to determine kinetic parameters such as the rate constants for the oxidation and the reduction of the redox system. In addition the shift of the flatband potential and of the corresponding energy bands at the GaAs-electrolyte interface could be followed during current flow. In the case of this redox system the potential dependence of the rates and of the interfacial current are in agreement with the theory on charge transfer kinetics at semiconductor electrodes. Above a Cu1+ concentration of 4 mM the oxidation current increased linearly with the concentration, and the second-order rate constant was determined to be k(v)(+) 5.10(-19) cm(4) s(-1).