Although the slaking of lime is a reaction of great antiquity, there have been few fundamental studies of its kinetics and mechanism. In this paper, the kinetics of the reaction are reported for lime samples in two different arrangements: rotating discs and powders. In the former case, the rate was measured by solution analysis; in the latter, by calorimetry. The rate of slaking of a rotating lime disc was approximately constant with time but depended on the disc rotation speed. In the presence of Ca2+ or OH− ions, the slaking rate was reduced. These results were accounted for by assuming that the slow step in the early stages of the slaking reaction was the diffusion of calcium hydroxide away from the reacting surface. Reasonable agreement between the measured slaking rates and those calculated on the assumption of slow calcium hydroxide diffusion was obtained. When powder samples were used, the rates of slaking could be fitted to the equation for a shrinking sphere model. It was shown that the effect of increasing the calcium oxide particle size or increasing either the time or temperature of preparation from calcium carbonate was to decrease the rate. Electron micrographs of the powdered lime before slaking were used to interpret these results. The effect of Ca2+ or OH− ions on the slaking rate of powder lime samples closely paralleled those of the rotating disc sample suggesting the reaction mechanisms are similar.