Based on experimental results and observations of electron microscope photographs, a model is proposed for the SO 2CaO reaction. The model simulates the closing of pore entrances as CaO is converted to CaSO 4. The size reduction and ultimately the closure of pore entrances due to product layer deposits cause an increase in the diffusion resistance of SO 2 through the pores. This in turn eliminates the availability of internal surface area for the reaction. Diffusion of SO 2 through the solid product layer is not included in this model, contrary to many previous models. The model is tested against experimental data without using any adjustable parameters, such as the product layer diffusion coefficient whose values ranged from 6 × 10 −9 to 2.5 × 10 −6 cm 2/s in the previous modeling efforts. Conversion data are obtained with precalcined limestone in a differential quartz reactor. The CaO particles studied are 13, 50 and 253 μm in diameter and the SO 2 concentration in SO 2-air mixture is kept at 3163 ppm throughout the runs. Observation of electron microscope photographs and excellent agreement of this model with experimental data suggest that the particle deactivation is due to an increase in the diffusive resistance of SO 2 through the pores and the subsequent incomplete utilization of the internal surface area. Based on this study, SO 2 diffusion through the closing pores and chemical reaction at the active interior surface are generally the dominant resistances to this reaction for the cases considered here as well as for many others.