Abstract: An apparatus and process for separating gaseous mixtures of molecules of different mass in which a rotating means is caused to eject a contiguous plurality of successive groups of molecules into an evacuated space to form a continuous stream of said mixture; the molecules of each said group of molecules are allowed to move in accordance with their thermal velocities for a predetermined period of time following ejection, thereby to allow each said group of molecules to form a generally spherical configuration the outer radius of which will be enriched, in molecules of lighter mass, relative to lesser radii; and a collector means co-rotating with said rotating means is used to collect molecules, which have been allowed to move for said predetermined period of time in accordance with their thermal velocities, from at least one desired portion of said stream.