Absolute measurements have been made of the electron impact ionization cross sections for the reaction N 2 + + e -> N3 + +2e and its isoelectronic partner C + + e + C 2 + +2e. The technique of crossed beams was employed and, at incident electron energies in excess of about 100 eV, the cross section for N2 can be expressed byand that for C by Measurements were extended to energies below threshold and the presence of metastable ions observed ; their influence upon the measured cross sections is discussed.The present experimental data is compared with Coulomb-Born calculations by Moores, classical binary encounter calculations by Thomas and Garcia, and a semiempirical estimate by Lotz.