Temporary anion formation in diimines bearing CH3, C(CH3)(3), and Si(CH3)(3) substituents is studied by means of electron transmission spectroscopy and ab initio calculations. The sizable reduction in the HOMO-LUMO transition energy caused by silyl substitution is found to be due not only to destabilization of the HOMO, as known from literature data, but also to stabilization of the LUMO, the mainly pi*(N=N) empty orbital.