The existence of two peaks in the energy spectrum of uranium was evidenced for the first time with SIMS during the sputtering of a UO2 sample as a function of the oxidation state of the surface. The surface oxidation state was quantitatively determined using the O18 method. These two peaks were attributed to two different surface binding energies of uranium. Since their relative intensities change as a function the surface oxidation state, the two surface binding energies must be associated with two different oxidation states of uranium. (C) 2011 Elsevier B. V. All rights reserved.