In a process for separating isotopes which comprises (1) irradiating a gaseous compound containing a mixture of isotopes with differing excitation energy levels with radiation of a first frequency which will selectively excite molecules of the compound containing a first isotopic species but not those molecules containing other isotopic species, and (2) subjecting the excited molecules to physical or chemical processes or a combination thereof whereby the first isotopic species contained in said excited molecules is separated from the isotopic species contained in the unexcited molecules, the improvement consisting of adiabatically expanding said gaseous compound to form a highly cooled supersaturated gas in which spectrum simplification has occurred, flowing said supersaturated gas through a region wherein said supersaturated gas behaves as a continuous medium, irradiating said supersaturated gas in said region with said first frequency radiation, irradiating said supersaturated gas in said region with radiation having a second frequency sufficient to induce photochemical reaction in those molecules selectively excited by said first frequency radiation, and separating the reaction product produced thereby containing the first isotopic species from those molecules containing other isotopic species before substantial condensation occurs.