Magnesium sulfate-water vapor is an interesting working pair of thermochemical materials for compact inter-seasonal heat storage at low temperature. Total dehydration of magnesium sulfate heptahydrate shows a theoretical storage energy density of 2.8 GJ·m⁻³. However, kinetic data are poorly studied up to now making use of this material difficult for a practical storage application. In the present work, a kinetic study of the dehydration of MgSO₄·6H₂O powder at low temperature (35 to 60°C) and at low water vapor pressure (2 to 21 hPa) is carried out using thermogravimetric analysis in isobaric-isothermal conditions. A mathematical model is developed for this bivariant system and validated representing the mechanism of dehydration: water molecules diffusion in the solid solution followed by transfer of these molecules from the surface to the atmosphere. The transfer of water molecules at the surface during dehydration is identified as rate-determining step. The fractional conversion and reaction rate of the dehydration reaction are calculated and compared to the experimental data.