The energy spectra of UO2 samples with slight hyper-stoichiometry were measured. In addition to the regular peak detected at 20 eV, it was observed that another peak at 80 eV would grow with the increasing stoichiometry. The relative intensities of the two peaks were used to determine the surface O/U ratio of the sample. We discuss how it would be possible to determine bulk O/U ratio from these results. Copyright © 2012 John Wiley & Sons, Ltd.