Abstract: Dehydration and recrystallization reactions of fine and coarse powders and also of massive samples of serpentine under isothermal heating conditions in air are followed by thermobalance measurements and by X-ray diffraction intensities. The rate of recrystallization to forsterite is shown to have an inverse relationship to the rate of dehydration. This result is interpreted in terms of the damage inflicted on the crystal structure of serpentine by the dehydration reaction; the more slowly this reaction occurs, the more readily is forsterite formed as a result of the topotactic relationship between forsterite and serpentine. The surface layers of particles and massive samples of serpentine which dehydrate readily appear to be highly disordered and consequently recrystallize to forsterite very slowly. The corresponding phenomena exhibited by kaolinite are discussed and compared with those by serpentine.