The equilibrium constants for the reaction of gaseous silicon monoxide with both oxygen and vapor separately have been computed from free energy data. The values obtained have been used to determine the ratio of partial pressures of silicon monoxide and silicon dioxide which exist in the gas phase in equilibrium with various partial pressures of the oxidizing gas. It is shown, by assuming that a gas phase interaction is predominant when solid silicon monoxide is evaporated in an oxygen or a water vapor atmosphere, that it is possible to predict the composition of the resulting film. There is reasonably good agreement between these predicted values and those found by experimental analysis. © 1962.