Photocurrent-voltage characteristics that show a nearly ideal dependence on the concentration of a nonadsorbed, outer-sphere redox acceptor (cobaltocenium) have been achieved with a novel photoelectrode structure that consists of a thin (30-50 Angstrom) epilayer of GaInP2 deposited on a thick (5000 Angstrom) p-GaAs epilayer. The thin GaInP2 layer produces nearly perfect passivation of the GaAs surface (resulting in a surface recombination velocity <200 cm/s), while at the same time permitting efficient electron transfer via field-assisted tunneling and/or thermionic emission, The electron transfer kinetics, as characterized by quenching of the GaAs photoluminescence, also shows a systematic acceptor concentration dependence. An unambiguous concentration,dependence of the photocurrent and electron transfer kinetics has not been previously reported at semiconductor-liquid junctions.