The (photo)electrochemical and etching proper-ties of n- and p-type In0.53Ga0.47As in an electrolyte solution (1.3 mol liter(-1) H2SO4) with and without H2O2 were studied by rotating-disk voltammetry, electrical impedance, and etch rate measurements. It is shown that H2O2 is reduced by a two-hole injection process. In0.53Ga0.47As is etched efficiently by H2O2 in a combined chemical/electroless etching mechanism, the dissolution intermediates of which appear to be mobile. The observed selectivity of H2O2 for In0.53Ga0.47As as compared to InP is explained on the basis of the present results. A comparison is made between the electrochemical and etching behavior of In0.53Ga0.47As and that of common III-V semiconductors such as InP and GaAs.