Abstract: A gas centrifuge and method for the separation of isotopic gaseous mixtures, particularly for the enrichment of uranium by the evaporative, concurrent-flow and countercurrent-flow principles using Taylor circular Couette motion. Gaseous isotopes either alone or mixed with a carrier gas, more particularly uranium isotopes in mixture with uranium hexafluoride carrier gas, are fed to a rotor assembly of a gas centrifuge which comprises two concentric cylinders which may be rotated at the same or at different angular velocities and in the same or opposite directions to create centrifugal forces sufficient to diffuse the heavier fraction of the gas mixture to the periphery of the assembly and the lighter fraction towards the axial portion of the assembly. The rotor comprises an inner, perforate, rotatable cylinder and an outer, continuous, smooth-walled, rotatable cylinder concentric with the inner cylinder and defining an annulus therebetween.