A variety of oxides doped with elements of lower valence acquire hydroxyl-type defects when exposed, at high temperature, to atmospheres containing water vapour. Since the hydrogen of the hydroxyl groups is mobile, the oxides display protonic conductivity and may be used as electrolytes in sensors, hydrogen pumps, fuel cells, etc. The extent to which protonic defects form depends mainly on the partial pressure of water vapour, temperature and basicity of the constituent oxides, while their mobility depends, among other factors, on the metal-oxygen bond length and bond energy. The defect equilibria that determine the protonic concentrations are considered, with emphasis on the regime of low oxygen partial pressure. The measurement of the thermoelectric power (TEP) and of the H+/D+ isotope effect in conductivity are discussed as a means of characterising the conduction process. (C) 2001 Elsevier Science B.V. All rights reserved.