Abstract: A method and an apparatus for the separation of isotopes from an atomic or molecular paramagnetic or polar beam of isotopic mixtures, by the utilization of the isotopic differences of said atoms as well as molecules, in the Electron Spin Resonance (E.S.R.) spectra and in other magnetic or electric resonances such as in the Nuclear Magnetic Resonance (N.M.R.) or Stark Resonance, is disclosed. The apparatus comprises means for producing a paramagnetic or polar particle beam of an isotopic mixture; means for changing the distribution of the m.sub.j values in said particle beam to at least one beam having a discrete m.sub.j value or values, where m.sub.