The channeling behavior of 1-MeV He+ ions and deuterons in U4O9 has been studied in order to provide information on the location of the excess oxygen in the fluorite structure of UO2. Wide-angle (Rutherford) scattering was used to study the channeling behavior with respect to the uranium sublattice, and the 16O(d, p)17O reaction was used to investigate the oxygen sublattice. A pronounced disorder in the oxygen sublattice was indicated by very poor channeling properties and strong dechanneling with increasing depth. The data are thus in conflict with the model incorporating the excess oxygen in the regular interstitial positions along the [Formula: see text] uranium rows; on the other hand, they are consistent with the more recent model of Willis in which the excess oxygen atoms are displaced ~ 1 Å from the regular interstitial sites and displace, in addition, some normal oxygen atoms. Channeling with respect to the uranium sublattice was found to be substantially the same as in stoichiometric UO2—but with a somewhat smaller critical angle, indicating that the uranium atoms may be displaced slightly (i.e. ~ 0.25 Å) by the presence of the excess oxygen.