In this paper, using the density functional theory, we present a consistent study of the thermodynamics of the Si-SiO2 system. This includes the formation of oxygen interstitials in silicon and of oxygen vacancies in silicon dioxide (cristobalite). Relations between calculated and measured quantities are analyzed. The composition dependence of the Si-Si and Si-O bond energies are discussed, which has implications on the modeling of the Si-SiO2 interface.