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The water phases in a MgSO₄·7H₂O crystal during heating were studied with the help of NMR. The thermogravimetric analysis (TGA) data showed that the heating rate has a strong effect on the dehydration process. NMR experiments showed that pore water, i.e., an aqueous solution of MgSO₄, was produced during dehydration. Additional NMR measurements indicate a water diffusion coefficient of this pore water that is 10 times lower than that of bulk water. We showed on the basis of a model that pore water is produced as the vapor release is not sufficient at crossing a phase transition temperature.