A (111) air-exposed surface of UO2 thin film (150 nm) 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 k(o) = 2 + x of a UO2+x oxide on the surface. It was found that initial surface (several nm) had k(o) = 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 180 s). Ar+ etching and subsequent annealing at temperatures 100-380 degrees 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 (similar to 28% of the basic peak) shake-up satellites 6.9 eV 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 k(o) approximate to 2.12 forms on the UO2 surface. (C) 2017 The Author(s). Published by Elsevier B.V.