The kinetics of the reaction between CaO and SO2 was studied by the random pore model, to accurately consider the porous structure of the solid reactant. The inherent reaction rate constants and rate dependency with respect to the SO2 concentration were determined from the initial slopes of the conversion-time profiles obtained by thermogravimetry. A power law concentration dependency was applied and then the first-order kinetics was verified. The product layer diffusivity was evaluated from the comparison of whole experimental conversion-time profiles with the model predictions.