We investigate reactions of peroxide etches on 〈100〉 surfaces of In0.53Ga0.47As by measuring the pseudodielectric function 〈ε〉 with spectroscopic ellipsometry. By comparing these data to calculations in the Bruggeman effective medium approximation, we find that residual 10–80 Å layers of low density oxide, amorphous arsenic, or mixed oxide–arsenic compositions are formed depending primarily on solution pH. The chemical identity of these overlayers is in accord with their known solubility dependence on pH. Etch compositions previously shown to reduce reverse- leakage currents in In0.53Ga0.47As photodiodes correspond to those for which arsenic precipitates at the surface.