Over the last few years, a method has been developed for obtaining ionization probability curves with essentially monoenergetic electrons. A retarding potential is applied to the electron beam to yield an energy distribution with a sharp low‐energy limit. By varying the retarding potential slightly, a new low‐energy limit of the distribution can be selected. The difference in the ionization produced in the two cases is ionization by those electrons with a small energy spread selected from the original distribution. By pulsing the electrons and ions, it is possible to eliminate the adverse effect of the ion‐drawout field on the electron energy. With this retarding potential difference (RPD) method, a detailed analysis of ionization probability curves is possible. A full description of this method is given in this paper with a discussion of its advantages and limitations. The mass spectrometer used in this series of studies is described, particular attention being given to a description of the ion source. The various electrodes of the electron beam slit system are described in terms of their influence on the electron energy, and on the shapes of ionization probability curves.