Surface hydroxyl groups on metal oxides are sites for adsorption of cations or anions from water, which in turn release protons or hydroxide ions to solution resulting in ion-exchange reactions. It is important to know the mechanism of the formation of surface hydroxyl groups to understand the ion-exchange capacities and properties of metal oxides. The hydroxyl groups are formed by dissociative chemisorption of water molecules, and it is generally considered that hydration and hydroxylation occur at exposed lattice metal ion sites on the surface as the lattice metal ions are strong Lewis acids. With this mechanism, the surface hydroxyl site density would be different for different oxide samples as the number of metal ions exposed is different depending on the metal ion valency and the lattice planes exposed. In the present study, the Grignard reagent method was applied to determine the surface hydroxyl site densities of a variety of metal oxide samples. The measured surface hydroxyl site densities were similar for different oxides with di-, tri-, and tetravalent metals. By considering this, a different mechanism of hydroxylation is proposed: Lattice oxide ions (rather than lattice metal ions) are exposed to aqueous solutions because of their large size and low polarizing power, and the surface oxide ions, which are strong Lewis bases because of insufficient coordination to the lattice metal ions, take up protons from water to form acid and base type hydroxyl groups. The maximum possible surface hydroxyl site density was calculated from the hydroxide ion size with the assumption that metal oxides have the structure of the closest packing of oxide ions. The measured values could be explained with the calculated value.