Summary: The crystallographic properties of the hydrated iron oxide obtained by the hydrolysis of FeCl3 are described. The characterizing X-ray powder pattern can be indexed on a tetragonal unit cell with a 10·48 and c 3·023 Å. This cell is shown to contain eight formula units of FeOOH but Cl- or other anions are necessary for the crystallization of the material The structure is that of hollandite or α-MnO2. The unit cell has been confirmed by electron diffraction from single crystals having dimensions about 5000 × 500 × 500 Å. and twinning, which produces hexagonal stars, has been studied. On heating, the structure breaks down and recrystallizes as α-Fe2O2, probably with an intermediate stage involving a spinel phase.