It has been shown in previous studies that a miscibility gap exists in the hypo-stoichiometric region UO2-PuO2-Pu2O3 with one phase poor in oxygen, and the other with an O/M (Oxygen to Metal ratio) close to 2.00. Data on the evolution of this region in temperature, especially in the vicinity of the oxygen content corresponding to the highest temperature at which the gap can be observed, is scarce. A high temperature X-ray diffractometer with a dedicated gas control setup was used to study the described region in-situ. We have observed reflections of the two cubic phases, with one increasing and the other decreasing in intensity during the thermal plateaus lasting up to 20 h. We compare the calculated lattice parameters with literature. We estimated the O/M evolution of our samples from a comparison of phase fractions values obtained by Rietveld refinement and calculations using the Calphad method.