The paper gives results of a study of the reaction of SO2 with Ca(OH)2 at conditions similar to those of commercial-scale bag filters: 19-74 percent relative humidity (RH), 30.4-95 C, and 300-4000 ppm SO2. The study was carried out in a bench-scale reactor with powder reagent Ca(OH)2 dispersed in silica sand. The gas phase was a mixture of N2, SO2, and water vapor. The effects of Ca(OH)2 leading, temperature, RH, inlet SO2 concentration, and additives were investigated. Of the additives tried (buffer acids, and organic and inorganic deliquescents), only the deliquescent salts improved Ca(OH)2 reactivity toward SO2. The improvement depends on the type and amount of salt and on the RH. The experimental data were modelled by a shrinking core model with zero order kinetics in SO2, using an empirical correlation to account for shape and surface roughness of the Ca(OH)2 particles. The diffusion coefficient of the SO2 through the product layer (De) increases linearly with RH and the amount of additive, and the kinetic rate constant (ks) increases exponentially with RH and the amount of additive.