The formation and growth of dislocation loops have been investigated under irradiation in the high-voltage electron microscope (HVEM) for the V-20Ti and V-3Ti-lSi alloys at temperatures ranging from 100°C to 350°C. In all cases, the loop number density increased, reaching saturation. The dependence of the saturated loop density on the temperature and the irradiation intensity showed that, in either alloy, the interstitials are deeply trapped by matrix impurities providing the loop nucleation sites at lower temperatures. In the case of the V-3Ti-lSi alloy, di-interstitials are expected to be the effective loop nuclei at higher temperatures. The temperature dependence of loop growth rate showed the effective vacancy migration energy of 1.36 eV and 1.22 eV for the V-20Ti and V-3Ti-lSi alloy, respectively. The present experiments showed some characteristic sample degradation (local bending, contamination) during in-HVEM annealing or irradiation suggesting a need for careful heat or environmental control.