The reaction between sulfur dioxide and limestone particles at high CO2 partial pressures (CaCO3 + SO2 + 1/2O2 --> CaSO4 + CO2), is investigated by using thermogravimetric analysis under isothermal conditions. A raw limestone is used. Different ranges of particles size, between 20 and 850 mum, and temperatures from 750-degrees-C to 900-degrees-C, are used in this study. By analyzing the dependence or time on fractional conversion, according to the shrinking core model, it is concluded that diffusion through the product layer is the controlling step for this reaction. The relationship between product layer diffusivity and temperature is given by D(e) = 0.0086 exp(- 146,500/RT), m2/s. The nature of the diffusion process and the mechanism of the reaction is discussed. By using the calculated rate parameters, the experimental conversion-time dependence is predicted.