The rate of reaction between magnesium and water vapor was studied in the temperature range between 425° and 575°C and in the water vapor pressure range between 31 and 208 mm Hg. The reaction took place according to the linear rate law. Rate constants were found to be linearly dependent on the water vapor pressure. The activation energy for the reaction was found to vary with both temperature and water vapor pressure, increasing as the temperature was increased and the water vapor pressure was decreased. This type of variation is explained on the basis of a change from a surface to a vapor phase reaction due to the very large increase in the vapor pressure of Mg metal in the temperature range between 500° and 600°C.