The kinetics of two redox systems were investigated on n-GaAs (100). The dependence of the cathodic Tafel lines on the donor density and on the concentration of the redox components was evaluated. The characteristic behaviour of the Br-/BrO3- system is the strong dependence of the onset of the cathodic current (flat band potential?) on the electron donor density. The cathodic Tafel slopes were independent of the donor density but dependent on the redox concentration. The cathodic currents in the Fe2+/Fe3+ system began at potentials in the bandgap and were a combination of hydrogen evolution and Fe3+-reduction. The results are interpreted by a potential dependent transfer from band pinning to Fermi-level pinning.