The reduction of Ce4+ at illuminated p-GaAs electrodes and of Fe(CN)(6)(3-) at illuminated p-GaAs, p-InP and p-GaP electrodes was studied. The common feature of these systems is the diffusion control of the dark current, which is due to hole injection by the oxidizing agent into the valence band. With "activated" electrodes under illumination using a photon flux smaller than the diffusion flux, the apparent quantum efficiency is nearly equal to zero and no H-2 evolution occurs. For higher light intensities, H-2 is produced with a quantum efficiency of one. This suppression of the hole injection can be explained by transfer of photoelectrons to the oxidizing agent. During activation by polarization in the negative potential region surface states are formed. These probably mediate the photoelectron transfer. The kinetics of the charge transfer processes were studied under constant illumination intensity and by Intensity Modulated Photocurrent Spectroscopy (IMPS).