Vacuum deposition of indium oxide film was carried out with an alternating supply of trimethylindium (TMIn) and H2O2 on GaAs substrates. Uniform thin films of polycrystalline In2O3 were obtained at 400-500-degrees-C. To study whether the In2O3 films could be used as a mask material for selective-epitaxy, we examined the deposition of GaAs onto the films by chemical beam epitaxy (CBE). No deposition of GaAs was observed on the films after irradiation with triethylgallium (TEGa) and AsH3 for 30 min below 600-degrees-C, which means that selective epitaxy of GaAs can be achieved on In2O3 masks. However, it was found that TEGa irradiation stimulates the sublimation of In2O3. Using X-ray photoelectron spectroscopy (XPS) analysis, the chemical composition of the oxide films was observed to change on TEGa irradiation from In2O3 to Ga2O3, for the first time. Probable chemical reactions responsible for In2O3 sublimation induced by TEGa irradiation are discussed. The selective epitaxy of GaAs on a patterned In2O3 mask and subsequent in situ removal of the mask by hydrogen plasma irradiation were demonstrated.