A good understanding of the spent fuel matrix (UO2) behavior under predisposal activities conditions is required for the proper performance assessment of a final repository. Hence, the oxidation evolution of UO2 under dry interim storage conditions, as a main predisposal action within the Spanish strategy, needs to be addressed. For this aim, in this work a detailed in situ Raman spectroscopy study of the surface oxidation of a UO2.00 disk heated in the presence of synthetic air at 573 K is presented. The spectra analysis required two previous studies. In the first one, UO2+x powder samples with controlled degree of non-stoichiometry were identified by thermogravimetric analysis and subsequently characterized by Raman spectroscopy. The equations obtained from this study enable estimating the oxidation degree of any UO2+x sample (for x < 0.20) at atmospheric conditions. The second one was performed in order to use these equations for the in situ experiments (at 573 K), since the shift of the bands due to temperature needs to be taken into account. Thus, the behavior of the Raman spectra as a function of temperature was analyzed and a correction term thereafter introduced in the initial equations.