X-ray powder diffraction studies have been made of the structure changes which occur on dehydrating the minerals diaspore and goethite, and also the synthetically-prepared “δ-Fe2O3.” For diaspore and goethite the changes are interpreted in terms of superstructures referred to a hexagonal, anion lattice sub-unit which is common to both the α-type oxyhydroxides and the anhydrous corundum or hematite structures. The ferromagnetic δ oxide, obtained by rapid oxidation of ferrous hydroxide, is shown to be a true oxyhydroxide of the form FeOOH. It transforms into goethite on heating at about 150°C but the change is incomplete due to a tendency to lose water directly to the atmosphere. An approximate structure analysis has been made, using the X-ray powder data for two relatively well-crystallized δ oxide preparations. It is concluded that the ferric ions occupy the interstices of the anion lattice in such a way that approximately 80 per cent. of the iron is randomly distributed in octahedral sites, while the remainder is similarly placed in tetrahedral sites. This cation distribution enables both the transformation to goethite and the magnetic properties of the oxide to be satisfactorily explained.