The decomposition of ethanol has been investigated on Ar+-sputtered surfaces of a UO2(111) single crystal. X-ray photoelectron spectroscopy (XPS) of the U 4f peaks after sputtering for 1 h showed the presence of two distinct oxidation states: U4+ (U 4f(7/2) at 380.2 eV) and U-0 (U 4f(7/2) at 377.4 eV). Upon ethanol exposure at room temperature, the peak at 377.4 eV was attenuated, indicating that U-0 sites were oxidized to Ux+i(x less than or equal to 4). The presence of a mixture of oxidation states on the surface influenced the reaction products observed during temperature programmed desorption (TPD). While ethylene and acetaldehyde desorbed in one temperature domain (at 560 K) from stoichiometric UO2(111), an additional desorption domain (at 475 K) was observed over the substoichiometric surface. The ratio of acetaldehyde to ethylene produced was different in the two temperature domains. While this ratio was near unity for the 560 K domain, it decreased to ca. 0.5 for the 475 K peaks on the substoichiometric surface. The lower temperature reaction channel is likely associated with surface oxygen vacancies, as it leads to greater oxygen abstraction, forming ethylene from surface ethoxide species. (C) 2000 Elsevier Science B.V. All rights reserved.