We have annealed Si‐SiO2structures at an elevated temperature in He(g) with SiO(g) added to the ambient. A SiO2decomposition reaction that normally forms large voids in a thin SiO2film during annealing in He(g) is not detected with the added SiO(g). Instead, chemically reactive SiC impurity sites which initiate the SiO2decomposition reaction are found segregated along crystallographic planes in the substrate at the Si‐SiO2interface. The mechanism and technological importance of this interfacial reaction are discussed.