The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range -500 to +500 mV (versus SCE). Cathodically cleaned UO2 specimens were anodically oxidized for I h and subsequently analyzed by XPS. Using published binding energies, the U(4f(7/2)) and O(1s) peaks were resolved into contributions from U-IV, U-V, U-VI, O-II, OH- and H2O. It was shown that over the potential range -500 to approximately +50 mV a thin surface layer of U-IV/U-V oxide (UO2+x) formed. At more positive potentials, a U-VI hydrated oxide (UO(3)(.)yH(2)O) was deposited on the electrode surface. At very positive potentials (greater than or equal to400 mV) the rapid anodic formation and hydrolysis of UO22+ led to local acidification in pores in the deposited UO(3)(.)yH(2)O layer and its subsequent re-dissolution. (C) 2004 Published by Elsevier Ltd.