It has been asserted previously that isotope separation in mass‐spectrograph‐type separators will cease when the ion density becomes so large that the time for Coulomb collisions between the two isotopic species becomes comparable to the transit time through the machine. It is shown here that the first effect of such collisions will be to increase,notdestroy, the separation, but that the current densities required for the effect are so far from the state of the art as to be regarded as inaccessible.