The chemical and electrochemical interactions between the (100) InP surface and acidic aqueous H 2 O 2 solutions were studied by etch rate, voltammetric, IMPS and electroluminescence measurements. Etching of InP by H 2 O 2 appears to occur at a very low rate through a chemical mechanism. Photocurrent enhancement caused by H 2 O 2 is observed both at the p-InP cathode and at the n-InP anode. Whereas the former effect is ascribed to reduction of H 2 O 2 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 H 2 O 2 so that they can more easily inject electrons into the conduction band of the semiconductor.