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We propose a complete model for the oxidation of silicon germanium. Our model includes the participation of both silicon and germanium atoms in the oxidation process and the replacement by silicon of germanium in mixed oxides. Our model is capable of predicting, as a function of time, the oxide thickness, the profile of the silicon in the underlying alloy, and the profile of germanium in the oxide. The parameters of the model vary with temperature, alloy composition, and oxidizing ambient. The model shows excellent agreement with published results, with model parameters following trends consistent with the physical phenomena hypothesized. The presence of germanium catalyzes both the silicon and the germanium oxidation rates, and all reaction rates increase with increasing temperature. The resulting effective oxidation rate is enhanced, with respect to the oxidation of pure silicon, at all germanium concentrations. Mixed oxides form only in the case of high germanium concentrations, but at high temperatures the rapid growth of a thick oxide results in a slowing of oxidant diffusion, and the oxide composition switches back to a pure silicon oxide.