The kinetics of the hydration of magnesia to produce magnesium hydroxide Is described by a kinetic model with no diffusive contribution, including an additional resistance to the reactive flux due to the transient variation of the porosity of the material during reaction. The proposed model has been applied to data from the literature and obtained by hydration of MgO samples with distinct physical characteristics. The model describes well the mechanism of hydration of powder and single crystal MgO in liquid and vapour water with temperatures varying from 35 degreesC to 200 degreesC.