Thermally grown silicon dioxide layers were irradiated with kilovolt electrons, and the equilibrium surface potential was measured by passing the secondary electrons through an energy analyser. The incident electrons at 5-keV energy penetrate for 3300 Å into a 5700-Å oxide film. The observed equilibrium surface potential is initially less than 1 V which indicates efficient induced conductivity; but after 10 C/cm2 it is -30 V (which indicates that the conductivity has diminished); and finally at 20 C/cm 2 it is less than 1 V again. Annealing reverses the process. A model is proposed. © 1969 The American Institute of Physics.