The shape of ionization efficiency curves (I. E. curves) obtained by essentially monoenergetic electron is studied by means of the threshold law. In the I. E. curves for N 2 + and CO + , some humps which are attributed to autoionization processes are found. These processes occur at 16.2±0.1 eV for N 2 + and at 15.6±0.1 eV and 18.4±0.1 eV for CO + . Though the shape of I. E. curve for Kr + obtained in this work is similar to that of Fox et al and Frost et al, it is difficult to interpret this shape by autoionization process only. The I. E. curve for A + shows large tailing of 0.50±0.05 eV which can not be explained by doublet of ground state of A + . The ionization potentials determined for rare gases and some diatomic molecules agree well with spectroscopic values and the previous electron impact data.