The effect of incremental ion bombardment on the surface layers of an aqua regia etched InP sample was studied by monitoring the components of the In 3d5/2and O 1sx‐ray photoelectron spectroscopy (XPS) lines as the sample was bombarded with low energy (1 keV) Ar+ions. The changes in the stoichiometry of the surface produced large shifts in the position of the In 3dand O 1slines that were not paralleled by shifts in the P 2pline. Analysis of these shifts indicated that the surface was covered with a mixture of indium hydroxide and indium phosphate, with the phosphate closer to the InP substrate. It is proposed that this layer structure is due to differences in the dissolution rates of the oxidation products in the acid etch and the effect of the distilled water rinse. It may be possible to alter the composition of such oxides by carefully tailoring the etch conditions to optimize the kinetics for the particular oxide phase required. The analysis of the XPS lines also showed that the InP substrate was damaged at very low ion doses, and finally decomposed by the ion beam. When the ion ‘‘cleaned’’ sample was exposed to oxygen, a different oxide system was produced which consisted largely of In2O3and InPO4[or In(PO3)x]. This model of the oxidized surface of InP is consistent with other measurements and we conclude that ion milling together with XPS and careful curve fitting can be used to find the nature of the thin oxides on InP.