The decomposition of SiO2 films on Si(100) during ultrahigh-vacuum anneal is studied in a scanning Auger microscope. Decomposition is found to be strongly enhanced by monolayer amounts of impurities deposited on the SiO2 surface. s- and p-band elements initiate decomposition via formation of volatile suboxides by surface reaction. In contrast, most transition metals decompose the oxide via laterally inhomogeneous growth of voids in the oxide, suggesting strongly that they need to diffuse to the SiO2/Si interface and there enhance oxide decomposition via formation of volatile SiO. It is hypothesized that existing defects in the oxide layer, enhanced by, e.g., film stress, permit metal migration to the interface. The oxide decomposition enhancement found for the transition metals is thought to be related to the electronic properties of those metals, namely, the density of states close to the Fermi level. A strong decrease in reactivity is found upon silicidation of the metal, supporting the above picture. © 1990 The American Physical Society.