Infrared spectra of haematite discs exhibited eleven maxima which were assigned to the OH-stretching vibrations of surface hydroxyl groups. The relative intensities of the infrared bands were dependent on the composition of the ferrigel from which haematite had been prepared by thermal activation in oxygen. Molecular water was weakly adsorbed on haematite and could be desorbed by evacuation at room temperature. Heat treatment of haematite in vacuum led to the formation of magnetite. Heat treatment of haematite in oxygen gave two effects. Firstly, surface dehydroxylation occurred and was reversed by the subsequent chemisorption of water at ambient temperatures. Secondly, irreversible sintering of haematite particles took place and led, not only to general decreases in the number of surface hydroxyl groups per unit weight, but also to changes in the relative proportions of different types of hydroxyl group. Variations in infrared spectra caused by differences in ferrigel composition or in the extent of sintering of haematite are attributed to variations in the external morphology of haematite particles and hence in the relative surface areas of the exposed crystal faces.