A critical assessment of oxygen chemical potential of UO 2+x , U 4 O 9 and U 3 O 8 oxide non-stoichiometric phases as well as of diphasic related domains has been performed in order to build up primary input data files used in a further optimization procedure of thermodynamic and phase diagram data for the uranium–oxygen system in the UO 2 –UO 3 composition range. Owing to the fact that original data are very numerous, more than 500 publications, a twofold process is used for the assessment – (i) first a critical selection of data is performed for each method of measurement together with a careful estimate of their uncertainties, (ii) second a reduction of the total number of data on the basis of a chart with fixed intervals of temperature and composition that allows a comparison to be made of the results from the various experiments. Results are presented for chemical potentials of oxygen with their associated uncertainties.