Radiative isotope separation (without lasers) has been achieved using isotopic differences in the rf spectrum of paramagnetic molecules in a supersonic nozzle beam. The combination of molecular‐beam magnetic resonance and supersonic molecular‐beam techniques can be used for isotope separation on a large scale. The principle was demonstrated experimentally for molecular oxygen. The vastly larger isotope spectral differences, the tunability of the spectra by external fields, and the use of the well‐developed technologies are promising starting points for this technique.