The direct sulfation of calcium carbonate was investigated in the temperature range of 600-900 degrees C under atmospheric pressure. Thermogravimetric analysis was employed to study the direct sulfation reaction. A gas mixture containing 3300 ppm sulfur dioxide in 5 vol.-% oxygen and 95 vol.-% carbon dioxide was passed through 3-5 mg of sorbent. The influence of particle size was studied using three different levels of particle average diameter. A variable diffusivity shrinking core model was applied to interpret the time-conversion data. The conversion dependent effective diffusivity in the porous sulfated layer was estimated based on the two mechanisms in series, i.e., pore diffusion and solid product layer diffusion. By applying the activation energy data for chemical reaction, available in the literature, the reaction rate constant, and the solid product layer diffusivity with Arrhenius type temperature dependency were estimated as adjustable parameters. The variation of fractional conversion with time can be reasonably predicted by using the developed model and the estimated parameters.