The kinetics of the dehydration process have been followed by vacuum microgravimetry and by direct optical microscopical measurement of the movement of product–reactant interfaces. Both single crystals and powdered samples were used. For the region where the rate of dehydration is constant and independent of α, the fraction decomposed, an activation energy of 17 ± 4 kcal. mole–1 has been evaluated by use of three distinct approaches. The Avrami–Erofeev–Kholmogorow equation is obeyed in the range 0·001 < α < 0·96, and it is concluded that the dislocations at which dehydration commences are randomly distributed throughout the bulk.