The kinetics of charge transfer processes between GaAs and various metallocenes in acetonitrile (ACN) have been studied. The oxidized as well as the reduced species are adsorbed at the GaAs surface. A model for the overall electron transfer process was derived that incorporates the adsorption of the redox system. The analysis of impedance spectra showed that the electron transfer from the conduction band of GaAs to the acceptor is extremely fast and that the overall rate of electron transfer is limited by the rate of thermionic emission from the GaAs bulk region to the surface.