The transformation of finely divided cubic to trigonal iron(III) oxide has been studied by gas adsorption, electron microscopy, X-ray and electron diffraction, and by magnetic measurements. The reaction is catalysed by water vapour, and appears to occur by the growth of nuclei as the result of diffusion. Accordingly, the surface area fell sharply when the phase change occurred, as found previously with the transformation in calcium sulphate. Certain aspects of the kinetics of the reaction were examined and are discussed.