The direct sulfation of limestone was studied in a quartz bench scale fixed-bed reactor with the technique of data deconvolution. The obtained results show that the direct sulfation of limestone has a two-period kinetic behavior: a short initial sulfation period with high but fast decreasing reaction rate and a subsequent period with product crystal growth with low-conversion rate. The transition from the first period to the second period is sharp and indicates clearly the start point of the period with product crystal growth. The transition point was determined by the concentrations of gases such as SO2, O-2, and CO2 and the temperature. The sulfation process in the initial stage of the period with product crystal growth can be described by the combination of the sulfation reaction at the gas-solid interface, diffusion of the product ions toward the product crystal grains, diffusion of carbonate ions toward the gas-solid interface to react, and the gradual shielding of calcite surface area by the product crystal grains. A simple and semi-empirical mathematical model derived on basis of this theory gives a satisfactory description of the experimental results up to a calcite conversion of about 0.5%. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1607-1616, 2011