The DTG curves for the dehydration of a number of preparations of Mg(OH)2were determined. Equations necessary to calculate energies of activation and rate constants from these curves were derived on the basis of a two-thirds-order process for dehydration. It was found that the energy of activation was about 5 kcal greater for naturally occurring than for precipitated Mg(OH)2. This was interpreted to mean that there was a difference in the energy levels of the two preparations, and therefore a difference in their apparent solubility products. Differences obtained in the DTG curves for four different preparations of Mg(OH)2indicated the probability of a series of energy levels for Mg(OH)2.