The influence of several material and processing parameters on the dielectric strength of thermally grown SiO2 films on silicon has been assessed. This was accomplished by statistically analyzing the breakdown characteristics of a large number of MOS capacitor structures, which had been fabricated in various ways. Although the results of this investigation are only qualitative, they clearly demonstrate that the effective breakdown strength of these films is strongly dependent on: Si02 purity, structural perfection and thickness; the presence of a passivating phosphosilicate glass layer; the presence and reactivity of the metal electrode; and, the duration of the post-metallization heat treatment. The morphology of certain micron-size defects which develop in the MOS structure during annealing is described in some detail, since these faults are apparently responsible for oxide shorting in some instances. © 1970, The Electrochemical Society, Inc. All rights reserved.