Summary: This process has been reinvestigated, mainly with X-rays, using an FeO-containing, fibrous variety (nemalite), and also a nearly pure specimen. The results confirm that the transformation is oriented, the c- and a-directions of the brucite becoming the normals to (111) and (11¯0) in the periclase. There is an intermediate, spinel-like stage; the spinel a-axis is parallel to, and twice as long as, that of the periclase. This stage is especially prominent when nemalite is heated in air or nitrogen, but is shown also by the nearly pure material. The nemalite shows also a further intermediate stage under certain conditions. A new hypothesis is proposed for the dehydration mechanism, in which the number of oxygen atoms per unit volume is ahnost unchanged in those parts of the crystal that are converted into periclase. The process is essentially one of cation migration, and the occurrence of the spinel-like phase is readily explained. Similar mechanisms possibly apply to the dehydration of other lamellar hydroxides and oxy-hydroxides (e.g. gibbsite, kaolinite) and of certain carbonates and other oxy-salts.