The effect of temperature and CO2 pressure on the kinetics of the reaction CaO + CO2°CaCO3 was studied. The reaction rapidly becomes diffusion-controlled as CaCO3 builds up on the surface of CaO. The pressure dependence of the diffusion-controlled reaction approximated that expected for a Langmuir adsorption isotherm. Temperature affected the extent of CO2 adsorption through θ, the fraction of surface sites covered, as well as the diffusion coefficient, D. The relation Rate =θKD describes the reaction where K is a constant containing geometrical factors, etc. The diffusion activation energy equals 29±6 kcal/mol.