The evolution of the oxide-overlayer's chemical nature on the surface of U-0.79 wt.%Ti alloy during vacuum annealing has been examined in situ by X-ray photoelectron spectroscopy (XPS). A specimen sheet of the alloy covered by oxide films is heated from room temperature to 700 degrees C in vacuum. It is found that the UO2+x outer oxide layer starts to be reduced to UO2 at 200 degrees C. Between 300 and 400 degrees C, an oxycarbide (UOxCy) layer is observed due to the reaction between UO2 and carbon. Above 500 degrees C, UOxCy decomposes and the surface covered oxide layer starts to be reduced to the metallic state, meanwhile, a thermal driven segregation of Ti to the surface is also observed. (C) 2015 Elsevier B.V. All rights reserved.