Changes in the interfacial chemical composition of n-GaAs(100) electrodes during cathodic reduction of H2O2 solutions (pH 0) at fixed potential are monitored in real time using in situ infrared spectroscopy and in situ spectroscopic ellipsometry. Under conditions in which the current density spontaneously oscillates, synchronous oscillations are observed in the thickness of a porous layer of solid arsenic hydride, with typical variations of a few hundred Angstrom. A physical and chemical model is proposed that accounts mathematically for the detailed time- and potential-dependent behavior and gives intuitive understanding of the chain of events occurring during an oscillation cycle. The periods of slow current rise correspond to the growth of the electroactive arsenic hydride phase, on which H2O2 is cathodically reduced. The current peaks coincide with the sudden dissolution of arsenic hydride, when the local potential reaches a value at which the phase is no longer cathodically protected, due to ohmic potential drop; surface H atoms are then replaced by OH groups, at which H2O2 adsorbs at a much higher rate. The model not only accounts for the current oscillations under potentiostatic conditions but also for the potential oscillations observed under galvanostatic conditions.