In2O3 thin films with preferred (4 0 0) orientation prepared by the spray pyrolysis method were studied by synchrotron radiation photoemission and ion scattering spectroscopes. O 1s, O2s, In 4d core level and valence band spectra were monitored at photon energies 660, 245, 150, and 73 eV to see their evolution with UHV treatments (heating, sputtering and exposure of oxygen). Reduction of the surface layer to nearly metallic indium was found with thermal treatment at T > 300 degrees C. This surface demonstrates high reactivity to reversible oxidation/reduction processes. This was evidenced by evolution of the O 2s core level peak and of the band gap emission intensity. In spite of such surface reduction it was found that within a probing depth of < 10 A the material displays spectral features characteristic of stoichiometric In2O3. We tentatively explain such behavior in terms of the In2O3 crystallographic structure and some conclusions relating to gas-sensing properties were made. (c) 2004 Elsevier B.V. All rights reserved.