Chemical species at the electrochemical interface between n-GaAs(100) and H2O2 in 0.5 M H2SO4 are identified by in situ infrared spectroscopy and in situ ultraviolet-visible ellipsometry. Under anodic conditions, H2O2 dissolves GaAs chemically, and the surface appears rough but clean of other phases. The sulfate concentration near the surface increases to compensate for the charge of the Ga3+ ions produced by dissolution. Under cathodic conditions, H2O2 is reduced electrochemically, bur chemical GaAs dissolution is never completely suppressed; both dissolution products are indirectly detected, the Ga3+ ions because they lead to an increase in the local sulfate concentration and HAsO2 because part of it is reduced cathodically, giving islands of a solid arsenic hydride, most likely As2H2. Variations in the amount of As2H2 and its surface coverage by adsorbed H and OH groups affect the H2O2 reduction rate. A kinetic model is proposed for the interfacial chemistry and electrochemistry. Numerical simulations reproduce the main features of experimental current-potential scans, including the presence of a negative slope region related to oscillatory behavior.