Transmission electron microscopy (TEM) experiments were performed, on a set Of UO2 thin foils following the implantation of 390 keV Xe3+ and 300 keV Cs2+ ions. TEM results reveal the nature and quantity of defects formed as a function of Xe and Cs ion fluences. These do not appear to be dependent upon the nature of the implanted ions. In addition to dislocation and dislocation loop formation, Xe and Cs aggregates were observed. At several Xe and Cs concentrations, the threshold temperatures for aggregate formation were determined. At concentrations of 0.4 at.%, Xe precipitation occurred above 870 K. The threshold temperature for Xe precipitation decreased to 670 K for the higher concentration samples (2 at.%) and to 770 K for Cs implanted samples at this concentration. No significant differences in the bubble size and densities were observed between the high concentration Xe or Cs samples. The aggregates formed were in the nanometer size range. (c) 2008 Elsevier B.V. All rights reserved.