The carbonation rate of 15-20 mu m, nonporous, calcium oxide crystals has been studied over a temperature range of 550-1100 degrees C and a CO2 pressure range of 1-11.7 atm. At temperatures greater than 600 degrees C, the carbonation rate decreases more rapidly with time than would be expected from diffusion through a uniform product layer and the activation energy is initially low but increases with conversion The product layer consists Of crystalline grains, and these product layer grains grow by coalescence from less than one mu m diameter to the approximate dimension of the particle. The carbonation rate can be described by a model where CO2 pressure-independent grain boundary diffusion and diffusion through-the carbonate crystals act in parallel. The relative importance of bulk diffusion through the product layer crystals increases with time relative to transport through the grain boundaries and has an effective activation energy of 57 kcal/mol. Diffusion through the crystal boundaries has low activation: energy and low dependence on CO2 pressure. At temperatures greater than 900 degrees C and times greater than 600 min, bulk diffusion through the product layer becomes dominant and the rate ap,roaches first order in CO2 pressure.