The rate of sulfation of a CaO surface is rapid at first, limited by the intrinsic kinetics, but slows down with increasing conversion as a consequence of the increased resistance to diffusion through the product layer. The objectives of this study are to determine the intrinsic kinetics and the product layer diffusion pate by minimizing the resistances to gas-phase pore diffusion, and eliminating complications due to pore filling. This is achieved by the use of nonporous CaO. A wide range of particle sizes are used to change the relative importance of the regimes in which the intrinsic kinetics and product layer diffusion control. The assumption of constant product layer diffusivity can then be tested and the variables that determine this diffusivity independently studied. Information on product layer diffusion can also be obtained from studies of porous particles after the pore mouths are all plugged and a uniform surface coating is obtained. This information on diffusion rate and intrinsic reactivity can then be combined with a geometrical model to describe the rate of reaction over the entire range of conversions and is particularly useful in treating the effect of particle size on conversion history.