We have applied high electric fields to films of SiO2thermally grown on silicon. The films were not metallized and high fields were obtained by depositing charge directly onto the surface from a corona discharge in air. After terminating the discharge the surface potential was measured, using a vibrating capacitor plate mounted near the free surface. Both the surface potential and the current through the oxide decrease with time, rapidly at first and then more slowly. When the current level has dropped to 10−9A/cm2the surface potential is changing by only a few percent per minute and the field across the oxide has nearly a steady value,Es. We measureEsand compare it with the dielectric breakdown fields that have been reported by others. We distinguish between the operational quantityEsand the breakdown field as usually reported since there appears to be no destruction of the sample in our experiments. We find thatEsis independent of the conductivity type and the doping level of the silicon, even when it is degenerate. When the free surface is charged negatively,Esis about twice as large as it is for positive charging.