The chemisorption of oxygen on clean polycrystalline indium surfaces is investigated by the combined techniques of x‐ray photoelectron spectroscopy (XPS) and the static mode of secondary ion mass spectroscopy (SIMS). Oxygen uptake is characterized by a moderately rapid formation of In2O3, with no preoxidation adsorbed O2phase discernable in the XPS spectra. A high binding energy O (1s) peak becomes prominent after one monolayer of In2O3has formed, which we ascribe to adsorbed oxygen on the oxide. Formation of In2O3continues into the bulk either via migration of the adsorbed oxygen through the oxide layer to the metal‐In2O3interface or by the outward migration of indium cations to the surface. Both the formation of oxide and the onset of adsorption are reflected in the secondary ion yield ratios InO−2/InO−and O−2/O−. The behavior of SIMS molecular cluster ion yields with oxygen exposure are consistent with recent theoretical calculations of the cluster formation process, in which considerable local atomic order of the surface is present in the cluster.