Most experimental data available in the literature on the high-temperature reaction between sulphur dioxide and limestone have been obtained in the form of an average degree of conversion (sulphation) of the solid limestone particle. Such data have been widely used as a basis for mathematical models of the reaction. It is, however, well known that a sulphated limestone particle is characterized by a highly non-uniform intraparticle conversion profile. Information on the intraparticle reaction is therefore desirable in order to improve the basis for mathematical models. Such information is presented in the following for a limestone constituted of non-porous grains. The product of the sulphation reaction under oxidizing conditions is demonstrated to be anhydrite II, not anhydrite III as commonly assumed in the literature. These two forms of calcium sulphate are of different molar volume, an important parameter in any detailed mathematical model. Pore size distributions have been determined before and after calcination and after sulphation. Calcination causes formation of a micropore structure and development of a bimodal pore size distribution. During sulphation the micropores become filled, or their entrances become blocked, but the macropore structure present in the uncalcined material remains. Intraparticle conversion profiles were determined in sulphated particles by energy-dispersive X-ray analysis. The degree of conversion decreases linearly from the particle surface inwards to a certain distance from the surface where it decreases to zero, forming a sharp front that moves inwards with increasing reaction time.