Models for estimating the sulfation of CaO at high temperature are presented. Short-residence-time sulfation is described by a pore size distribution model and long-residence-time sulfation by a particle expansion model. The calculations made agree well with experiments carried out on three different limestones in a TGA, a volumetric reactor and an entrained flow reactor. The pore size distribution model explains the effects of particle size, pore size distribution and partial pressure of SO2, suggesting these three factors to be the most important for CaO conversion. For particles larger than 1-2 mu m, pore diameters of 50-300 Angstrom are desirable. When large particles or long-residence times are used, as in fluidized-bed combustion, the particle expansion model shows the particle size and the sorbent type to be the main factors affecting the reaction.