The interfacial properties at a rotated p-GaAs/GaInP2 electrode were investigated in acetonitrile solutions of metallocene redox couples. Current-voltage and interfacial capacitance measurements display minimal band edge movement with changes in solution potential, indicating a well-passivated surface with few surface states within 1 eV above the valence band edge. However, analysis of the shape of the current-voltage curves and the Mott-Schottky plots obtained in the light indicates a negative potential shift in the position of the band edges upon illumination. The extent of band edge movement depends on the concentration of the oxidized form of the redox couple in solution. Greater band edge movement upon illumination was observed at low concentrations, while less movement was observed at higher concentrations, indicating the accumulation of photogenerated electrons. Possibilities for the existence of the accumulated charge in states just below the conduction band are discussed.