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The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600-700°C under 25 MPa were carried out for up to 1000 h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni-Fe/Ni-Cr spinel oxide, Ni/Co hydroxide and TiO 2 and an inner layer of a mixture of NiCr 2 O 4 and Cr 2 O 3 while the innermost layer is made up of Cr 2 O 3 . The MoO 3 can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.