Separating gaseous isotope mixtures

In an improved process for separating a gaseous isotope mixture having the general molecular composition X.sub.n1.sup.(1) X.sub.n2.sup.(2) where a constituent X.sup.(k) can be replaced by an isotope, the improvement which comprises injecting laser light excitation irradiation into a location selected from the group consisting of the top and the bands of the fundamental and combination vibrations, and the vibration band edges of the electronic transitions associated with the isotopes comprising said mixture, said excitation irradiation being supplied by a laser source selected from the group consisting of a tunable laser and a pumping laser followed by a secondary tunable laser.