Current density vs potential, open-circuit voltage vs temperature, and differential capacitance vs potential measurements have been used to show that n-GaAs and n-Al x Ga 1-x As electrodes exhibit partial Fermi level pinning in contact with CH 3 CN over a wide range of redox potentials. Despite a change of over 1.2 V in redox potential of the solution, the open-circuit voltage only changed by ∼300 mV. The slope of the opencircuit voltage vs redox potential of the solution was typically 0.33-0.44. Differential capacitance vs potential data also yielded a barrier height change of less than 300 mV for over 1.2 V change in the redox potential of the solution. The dependence of the current density vs potential behavior of n-GaAs/CH 3 CN-ferriceniumferrocene +/0 on variables such as the illumination intensity, dopant density of the semiconductor, concentration of redox acceptor in the solution, crystal face, electrolyte, and cell temperature was evaluated. The resultant kinetic data indicate that surface-state recombination is the dominant recombination mechanism at these interfaces, which are capable of producing an open-circuit voltage of 0.83 V at a short-circuit current density of 20 mA cm -2 , as well as energy conversion efficiencies of > 10%. X-ray photoelectron spectroscopy investigation of n-GaAs confirmed surface changes were induced by electrochemical operation of n-GaAs electrodes in CH 3 CN-cobaltocenium-cobaltocene +/0 electrolyte. The presence of Fermi level pinning and the existence of changes in n-GaAs and n-Al x Ga 1-x As electrode surfaces when these electrodes are in contact with CH 3 CN-cobaltocenium-cobaltocene +/0 electrolyte complicates the extraction of k et values from the steadystate current density vs potential behavior of n-GaAs or n-Al x Ga 1-x As/CH 3 CN contacts.