The microstructure evolution of ion-irradiated UO2 was evaluated by in situ Raman spectroscopy. Transmission electron microscopy (TEM) was also conducted for selected irradiation conditions. The UO2 samples were irradiated with single- (900 keV I ions) or dual-ion beams (900 keV I and 27 MeV Fe or 36 MeV W ions) with different ratios between the high- and low-velocity ion flux (R). The analysis of the damage build-up shows that the electronic excitations deposited by the high-velocity ions affect the formation and evolution of defects induced by nuclear energy loss. In addition, the kinetics of dislocation evolution are more advanced when the flux ratio promotes electronic excitations. Consequently, a higher R value favors electronic excitations, leading to greater microstructural evolution.