The discovery of a method and the invention of a system for obtaining very high selectivityand dissociation of the desired .sup.235UF.sub.6 isotope in the Molecular Laser Isotope Separation (MLIS) process of the Uranium Hexafluoride (UF.sub.6) isotopes, in a single highly selective step, is described. The principle of the process and the concept of the invention are very simple: At temperatures below 100 K., and. preferably in the region of 60 K, nearly all the molecules of the expansion supercooled UE.sub.6 gas are in the ground state enabling the principles of the invention to be practically applied without-any interference from other inherent, processes. Then the frequency of the selecting laser must be at 628.527 cm.sup.1, or very close to it, for a three-photon absorption resonance with the [m(A.sub.2):(3V.sub.3)] sublevel of the third energy excitation state of the desired .sup.235UF.sub.6 isotope. The fixing of the frequency of the selecting laser is the first basic step of the invention. The second basic step is to increase the pumping intensity of the selecting laser to a. level at which thethree-photon absorption resonance with the [m(A.sub.2):(3V.sub.3)]sublevel, of the desired .sup.235UF.sub.6 isotope is established, elevating the molecules of the desired isotope .sup.235UF.sub.6 to the third energy excitation state. This is achieved through the power broadening at the fundamental and the second energy excitation level as the pumping intensity of the selecting laser is increased and as a consequence of the proximity of these levels to the pumping frequency. There is an intensity range for the selecting laser within which the molecules of the desired .sup.235UF.sub.6 isotope can be selectively elevated to the third energy level through the establishment of a three-photon absorption resonance without disturbing the molecules of the unwanted, isotope .sup.238UF.sub.6, leaving them unexcited. The selectively excited molecules of the desired .sup.235UF.sub.6 isotope are then driven to dissociation through, the higher vibrational levels of the v.sub.3-vibrational mode and. the quasicontinuum of energy states, by a simultaneously applied dissociating laser whose exact intensity and optimum frequency can again be experimentally determined, or by any other dissociation or separation-process following the original excitation of the .sup.235UF.sub.6 molecules to the third energy excitation state (3v.sub.3) through three-photon resonance with the [m(A.sub.2):(3V.sub.3)] sublevel. The process is unique in that it can be applied, to the treatment and separation of the desired .sup.235UF.sub.6 isotope from the Tails percentages of any isotope separation proce