Introduction: "The extensive use of uranium hexafluoride in the Nier-type mass spectrometers, where the ionization is produced by slow electrons, made a knowledge of the ionization potentials of the various ions desirable. This information was not available and work was accordingly begun to secure such data. The first measurement was made with a VGIA ion gauge and has been described in TEO report CD-310. With this apparatus it was not possible to identify the ionization products and it was presumed that the value reported was for the UF5 ion. When time and equipment became available, the work was continued and the critical potentials of the singly-charged ions determined."