[en] The method of separation described consists in producing a beam of positive or negative primary ions of at least one compound of the element of interest, in accelerating said ion beam in order to pass said beam through a predetermined number of collision boxes which are open at both ends and placed one after another in series, in successively initiating within each collision box, by inelastic collisions of said ions with the molecules of a neutral target gas with which said boxes are filled at low pressure, the dissociation of a given percentage of the primary ions into at least two fragments such that one fragment is a secondary ion which appears in the form of at least two different isotopic species with respect to the element to be separated, and in choosing the potential V₂ to which said collision boxes are brought in order to trap preferentially therein one of the abovementioned isotopic species which is caused to condense within each collision box