Abstract: The invention relates to an isotopic separation process.This process consists of exciting molecules A by the absorption of a pulsed light beam, then exciting molecules X present in several isotopic forms up to their dissociation by a vibrational transfer between molecules A and X. Molecules A have a dissociation energy higher than that of molecules X. The duration and energy of the light pulses are such that the absorption time by molecules A is below the excitation time of molecules X. The temperature conditions are such that the thermal width of the vibration lines is at least close to the isotopic shift between the resonance lines of the two isotopic forms.