Turbo-separative methods and apparatus for separation of mixtures of gaseous materials having different molecular weights, particularly gaseous isotope mixtures such as mixtures of UF.sub.6, in which differential sedimentation velocities of the components to be separated are established in a laminar boundary layer of the gas adjacent a hydraulically smooth, porous blade surface, and in which a predetermined amount of the boundary layer flow is conducted through the porous surface to stabilize the boundary layer and to provide a higher density gas fraction. The velocity of the gas conducted through the porous blade surface should not exceed the sedimentation velocity of the gas species to be separated.