A novel technique is described which was used to study the intrinsic breakdown mechanism in films of thermal SiO2in the thickness range 30–300 A˚. It was determined that high‐field and high electron injection current conditions existing in the films just prior to breakdown result in the generation of a very high density of defects which behave electrically as stable electron traps. These traps are most likely generated close to the injecting electrode. The internal field in the oxide due to trapped electrons can approach 3×107V/cm which appears to be the maximum field strength which Si‐O bonding can withstand. At all temperatures between 77 and 393 °K, the breakdown mechanism is intimately related to the rate of generation of the electron traps. No evidence was found to support the impact ionization breakdown model. The technique is also described as a tool for yield measurements, with important implications for long‐term reliability of MOS IC’s.