AbstractCalcium oxide samples differing widely in surface activity (5–100m.2g.‐1) have been ‘wet’ hydrated at room temperature using water, pure or diluted with acetone as an inert liquid.Rates of hydration and changes in surface area and lattice structure indicate that ‘wet’ hydration proceeds by a mechanism similar to that found earlier for ‘dry’ hydration by water vapour, i.e., hydration proceeds inwards from the outside of each calcium oxide particle by an advancing interface mechanism. In each case, the more active limes become aged (agglomerated) rapidly, but when the less active limes are ‘wet’ hydrated the number of crystallites generally increases much more than for ‘dry’ hydration; cleavage of the calcium hydroxide formed on the surface of the oxide is therefore more extensive, in keeping with the faster hydration. Slow ‘wet’ hydrations using 1% water more closely resemble ‘dry’ hydrations in that the number of crystallites increases only over their later stages. Ultimately, all the ‘wet’ hydrated samples age on prolonged contact with water, their crystallite growth showing some relationship to that of calcium hydroxide precipitated fr