Hydroxyl groups on the surface of metal oxides are amphoteric in character. From experiments with anatase, rutile, η-alumina, α-Fe2O3,CeO2, and SnO2, half the OH groups are acidic in character, the other half are mainly basic and may be exchanged for other anions. This behaviour is explained by the structure of the hydroxylated surface. Acidity and basicity depend on the nature of the oxide. Several reactions are described for the determination of total OH content as well as the quantities of acidic and basic OH groups.