The separation of a binary gaseous mixture of uranium hexafluoride 238UF6 with different light components in a high-speed centrifuge, intended for separating heavy isotopes, is examined. The mass of the light impurities is varied in the range M 1 = 0.02–0.349 kg/mole. It is shown that as the impurity mass decreases the structure of the flow fields in the centrifuge rotor changes considerably. If in the case of a mixture of heavy isotopes convective transport has a determining effect on the concentration, for a mixture with a large difference of the mass of the components radial barodiffusion in a centrifugal field becomes primary, which results in the appearance of a maximum of the separation factor at M 1 = 0.127 kg/mole.