The chemical and electrochemical interactions between the (100) InP surface and acidic aqueous H2O2 solutions were studied by etch rate, voltammetric, IMPS and electroluminescence measurements. Etching of InP by H2O2 appears to occur at a very low rate through a chemical mechanism. Photocurrent enhancement caused by H2O2 is observed both at the p-InP cathode and at the n-InP anode. Whereas the former effect is ascribed to reduction of H2O2 by the well-known current-doubling mechanism, for the latter effect a reaction mechanism is proposed in which intermediates of the photoanodic dissolution reaction of InP are modified by H2O2 so that they can more easily inject electrons into the conduction band of the semiconductor.