The decomposition of oxide films of 50 to 500 Å on Si(100) during ultrahigh vacuum anneal has been studied in a scanning Auger microscope. The decomposition of the oxide occurs locally, in that voids form in the oxide and grow laterally with time and temperature, leaving the oxide areas in between unperturbed. Void growth kinetics data are derived from in situ absorbed current and secondary electron imaging during vacuum anneal. The growth of the void diameter is found to be linear in time with an activation energy of 2.0±0.25 eV. This implies that oxide decomposition rates after the void nucleation phase are dominated by chemical reactions and/or diffusion processes near the circumference, not by the nature of the defect which nucleated the void.