An apparatus for separating a gas mixture into a heavy component and a light component, especially for the enrichment of a gas mixture in uranium 235 in which the uranium isotopes are provided in the form of uranium hexafluoride, comprises a cascade of centrifuge stages each formed by a pair of spaced-apart disks surrounded by a housing and driven by a common shaft from a motor. The relatively narrow passage between each pair of disks opens into a greater space which cooperates with the gas along the periphery of the disk as held in place by the housing to form a boundary layer with the heavy component tending to diffuse out of the layer. Means is provided to collect the light component in the boundary layer.