The origin of current and potential oscillations during hydrogen peroxide reduction on n-type GaAs semiconductor electrodes is investigated. Impedance measurements clearly demonstrate that the current oscillations are caused by a negative impedance in conjunction with the electrolyte or external resistance. By considering a simple kinetic model which combines earlier mechanistic models proposed for the H2O2 and H+ reduction on GaAs, it is shown that oscillations can occur only if the negative impedance is caused by an anomalous dependence of the band bending on the electrode potential, i.e., if an increase of the Fermi level of the n-type semiconductor results in an increase in the band bending. This effect is assumed to be caused by a hydride layer formed electrochemically during the concomitant proton reduction. Impedance measurements confirm the role of the band-bending anomaly.