Measurements of the cluster size (n) distribution of secondary (UOx)+n ions from sputtering of uranium dioxide (UO2) by Ne8+, Ar8+ and Xeq+ ions (q=10, 23) at fixed kinetic energy (81 keV) have been performed with a time-of-flight mass spectrometer. The cluster ion yields Y follow a power law Y(n)∼nδ with -2.1<δ<-1.5. This is in contrast to a statistical recombination of the constituents upon ejection, but in agreement with the predictions of collective ejection models. Such a power law was also observed in the electronic stopping regime with MeV/u ions. The exponent δ is found to decrease with increasing projectile mass (and thus total sputter yield) at fixed kinetic energy. The ratio of emitted ionic clusters to monomers varies from 3 to 4.5 depending on the projectile. The contribution of positive ions to the total sputtering yield amounts to about 0.03%.