UO2 and U4O9 single crystals were investigated by means of the 4He backscattering/channelling technique and the use of the 3032-keV elastic scattering resonance on 16O. Angular scans obtained for U4O9 were interpreted in the framework of a recently published U4O9 model. It was shown that the U4O9 channelling data can be explained by the aggregation of some O atoms in cuboctahedral clusters, and the displacement of other (U and O) atoms from the UO2-type sites. Basing on the combined use of the channelling technique and Monte Carlo simulations we suggest that the displacements of U atoms from their fluorite sites are larger than those determined by the neutron diffraction analysis.