The generation of structural and electrical defects in Si/SiO2 structures upon high temperature annealing by the oxide decomposition reaction Si+SiO2→2SiO ↑ has been studied using scanning electron microscopy (SEM) and ramped current-voltage measurements. The SiO decomposition is nucleated at crystalline defects in the substrate and results in the formation of voids in the oxide. The voids grow laterally with annealing time, independent of the nature of the defect. Prior to the formation of physical voids in the oxide, defects become electrically active, leading to low field dielectric breakdown. The breakdown degradation is prevented when the O2 pressure in the annealing ambient is sufficient to reverse the decomposition reaction by reoxidizing the SiO product at the interface. © 1987.