The mobilities of mass-identified U+ and Hg+ ions in helium have been determined in a drift tube-mass spectrometer apparatus. For uranium ions a reduced mobility value μ0=(16.0± 0.5) cm2/V.sec is obtained at 305°K and a standard gas density of 2.69× 1019cm-3. The mobility of mercury ions is μ0=(19.4± 0.5) cm2/V.sec at 292°K, in agreement with two previous determinations. The effect of fast ion injection in drift mobility measurements is discussed, and a new technique to circumvent these problems is described. The results are compared with existing theories of ion mobilities.