Under typical conditions found in Pressurized Fluidized Bed Combustion (PFBC), the calcination reaction of limestones is thermodynamically inhibited, and the sorbent reacts with SO2 by a direct mechanism. Direct sulfation reactivity of different Spanish sorbents was studied by high-pressure thermogravimetric analysis. We have found that the physical structure of the surface of the particles influences the sulfation behaviour of the sorbents. Total pore volume and pore surface area correlated well with the reactivity of the sorbents. Temperatures between 800 degrees C and 925 degrees C, and pressure between 12 and 25 bar, promoted an increase in reactivity, while the gas composition had no effect when changed from 15% CO2: 3% O-2: 0.5% SO2: balance N-2 to 12% CO2: 7% O-2: 0.5% SO2: balance N-2. We found that the unreacted shrinking core model could not satisfactory described the sulfation reaction under the conditions studied. The expression of the product layer diffusion coefficient as an exponential function of the conversion led to a modified unreacted shrinking core model which described the sulfation reaction more accurately.