The reaction brucite <--> periclase + water has been reinvestigated using pressure and temperature quenching techniques. It was observed that the dehydration curve obtained by temperature quenching and the hydration curve obtained by pressure quenching are about 40 degrees C. apart at elevated pressures. It is proposed that the difference between the 2 curves is due to the formation of an intermediate phase, probably corresponding to a distorted periclase. This phase rehydrates rapidly during the temperature quenching to form brucite. Because of this rehydration phenomenon, which is not dependent on the type of quenching, the brucite hydration curve is shifted to higher temperatures.