We studied in-situ the interaction of sodium metal with UO_2+x (0 ≤ x ≤ 1) using thin films prepared by sputter deposition on Au substrate. X-ray Photoelectron Spectroscopy (XPS) and Grazing Incidence XRay Diffraction (GIXRD) characterized films before and after interaction. The results show that sodium does not reduce stoichiometric UO₂ at room temperature. Plasmon loss peaks, observed at the Na1s photoemission (PE) line, are characteristic of metallic sodium particles, and point to a weak interaction between sodium and UO₂. The oxidation of the sodium at room temperature takes place only on hyperstoichiometric uranium dioxide films. Indeed, sodium deposition on UO_2+x (0 < x ≤ 1), eventually results in the complete reduction of U(VI) down to U(IV). Molecular and atomic oxygen affect differently the oxidation of uranium and sodium. The wetting of the gold by the sodium is much more important when sodium is oxidised by atomic oxygen. This leads to a shift of the Na1s core level peak, while molecular oxygen gives a peak broadening without shift. Atomic oxygen seems to play the same role as the interstitial oxygen present in the UO_2+x film. The oxygen dissociation may be the limiting step of sodium oxidation on gold and on UO₂. Heat treatment of sodium on UO₃ at about 773 K leads to the formation of the NaUO₃ film as demonstrated by a quantitative analysis by XPS and GIXRD. The results show also that once U(V) is formed, it stays stable up to at least 973 K, the temperature at which shrinkage of the film is observed.