A (111) air-exposed surface of UO2 thin film (150nm) on (111) YSZ (yttria-stabilized zirconia) before and after the Ar+ etching and subsequent in situ annealing in the spectrometer analytic chamber was studied by XPS technique. The U 5f, U 4f and O 1s electron peak intensities were employed for determining the oxygen coefficient kO=2+x of a UO2+x oxide on the surface. It was found that initial surface (severalnm) had kO=2.20. A 20s Ar+ etching led to formation of oxide UO2.12, whose composition does not depend significantly on the etching time (up to 180s). Ar+ etching and subsequent annealing at temperatures 100–380°C in vacuum was established to result in formation of stable well-organized structure UO2.12 reflected in the U 4f XPS spectra as high intensity (∼28% of the basic peak) shake-up satellites 6.9eV away from the basic peaks, and virtually did not change the oxygen coefficient of the sample surface. This agrees with the suggestion that a stable (self-assembling) phase with the oxygen coefficient kO≈2.12 forms on the UO2 surface.