The kinetics of hydration of magnesium oxide (MgO) powder to form magnesium hydroxide (Mg(OH)(2)) were measured using isothermal calorimetry at different temperatures, and the morphology of the powders before and after hydration were examined. The hydration kinetics of light-burned MgO exhibit a hydration rate peak similar to that of portland cement hydration, whereas the hydration kinetics of hard-burned MgO are comparatively slower at the same temperature, and exhibit a continuously declining hydration rate after the first several minutes of reaction. The hydration kinetics of both light-burned and hard-burned MgO can be fit using a boundary nucleation and growth model that has previously been applied to the hydration of portland cement and tricalcium silicate. Activation energy values for MgO hydration were determined from the fitted rate constants and were also measured directly using small temperature excursions according to a recently proposed method. For light-burned MgO the resulting values are in good agreement and indicate a value of 77 kJ/mol. For the hard-burned MgO the activation energy values vary considerably depending on temperature and how the activation energy is measured, but are always lower than the value obtained for the light-burned MgO.