The growth of thin In oxide layers on Si(111) substrate surfaces by reactive evaporation of In in oxygen atmosphere and the formation of the In oxide-Si interface under ultrahigh-vacuum conditions has been studied by Auger electron spectroscopy, electron-energy-loss spectroscopy, photoemission with use of synchrotron radiation, and inverse photoemission. Oxygen pressure and substrate temperature were varied as the parameters of the reactive evaporation. The combined electron spectroscopic results indicate that for In deposition onto room-temperature substrate surfaces and PO2<5×10 -5 mbar the interfacial layer contains metallic In and oxidized Si, but that for higher PO2 the metallic In concentration at the interface is reduced. The oxidation of In appears to be enhanced away from the In-Si interface, and for PO2≳5×10-5 mbar the oxide stoichiometry approaches that of In2O3. At elevated substrate temperature (250 °C) the growth rate of In oxide layers is reduced as compared to room temperature, but the oxidation yield at the interface is enhanced. For PO2=2×10-4 mbar and 250 °C substrate temperature the growth of a uniform near-stoichiometric In oxide layer is indicated.