A mathematical model developed accounts for the multiple rate processes involved in the reaction of solid CaCO3 or Ca(OH)2 with SO2 at high temperatures in the combustion environment. The model, based on the grain-subgrain concept, considers the concomitantly occurring calcination, sintering and sulfation reactions and their interactive effects on pore structure and reaction kinetics. It incorporates internal diffusion, reaction, and product layer diffusion in simulating the calcination of the CaCO3 grain and subsequent sintering and sulfation occurring on the CaO subgrains. It is the first sulfation model to incorporate the true mechanism of diffusion through the solid product phase: the solid-state ionic diffusion of Ca2+ and O2− ions in a coupled manner through the nonporous CaSO4 (Hsia et al., 1993, 1995). Its predictions are compared with the random pore model (Bhatia and Perlmutter, 1981a) and the grain model (Szekely and Evans, 1971) using experimental CaO sulfation data from the literature as well as short-contact-time CaCO3 and Ca(OH)2 sulfation data reported previously. Mahuli et al. (1997) discussed a high reactivity modified calcium carbonate synthesized by optimizing the pore structural properties. This modified CaCO3 can convert 70–75% of sulfation within 0.5 s, which is substantially higher than any other sorbents reported for similar particle size and reaction conditions. The model is used to predict the calcination and sulfation kinetics, as well as to simulate the surface area evolution of the modified CaCO3, which provides further insights into its exceptional reactivity.