The oxidation of six (U1–y,Puy)O2–x powdered samples, with y = 14, 24, 35, 46, 54, and 62 mol %, was studied in situ by high-temperature XRD at different temperatures. The samples were first reduced at 1500 °C to an oxygen to metal ratio (O/M) lower than 2.00. Afterward, the temperature was rapidly decreased to a value between 200 and 800 °C, which was followed by a long isothermal plateau during which diffraction patterns were recorded continuously. We observed structural changes in the material as it oxidized, in some cases passing through a biphasic domain. We discuss the influence of phase separation on the formation of a passive layer and the kinetics of phase transformations. It was found that entering the biphasic fcc1 + fcc2 domain will inhibit the formation of a passive layer. We hypothesize that the reason behind this behavior is the dependence of oxygen diffusion on the concentration of oxygen defects