We report on the development and application of a quasi-chemical thermodynamic model of metal evaporation (desorption) from Si surfaces during high temperature processing, specifically a high temperature wafer process referred to as hydrogen annealing (or baking). We show that significant depletion of many metals can be expected during such a process. This is an additional benefit of hydrogen annealing, providing another mechanism for improved defect levels in MOS gate oxides grown on hydrogen annealed wafers.