A finite rate kinetic study of magnesium reaction in vitiated oxygen and steam atmospheres at different pressures was conducted using the SPIN application of the commercial Chenikin package as an equation solver. This work focused on a bath reactor configuration that was more akin to the systems of interest in water-breathing undersea power plants, rather than on the type of droplet combustion of interest to the rocket community. Although previous work presumes that the surface temperature of the magnesium is very near its boiling point, the present work shows that this is not necessary for a vapor phase reaction to exist. The Hertz-Langmuir equation for magnesium evaporation combined with an appropriate reverse reaction rate was shown to adequately approximate evaporation in a combusting environment. It also permitted simulation of vapor-phase combustion for low magnesium surface temperatures and enabled predictions of ignition behavior that were reasonably close to measurements in the literature. Several kinetics models for the magnesium oxygen kinetics from the technical literature were examined and compared with pool reaction data from the literature as well. There is little finite rate information related to the behavior of magnesium/steam reactions, consequently, a macro-kinetic expression for the reaction with water vapor was developed using simple hard-sphere collision theory for the kinetics. The effect of steam dissociation on the reaction process was investigated and found to be significant for lower pressure combustion. Calculations were also performed in simulated atmospheres modified to mimic vitiated or partially consumed states.