Sorbents highly reactive towards SO2 have been prepared from iron blast furnace slag and hydrated lime under different hydration conditions. The reaction of the dry sorbents with SO2 has been studied under the conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction was well described by a modified surface coverage model which assumes the reaction rate being controlled by chemical reaction on sorbent grain surface and takes into account the effect of sorbent Ca molar content and the surface coverage by product. The effects of sorbent preparation conditions on sorbent reactivity were entirely represented by the effects of the initial specific surface area (So) and the Ca molar content (M-1) of sorbent. The initial conversion rate of sorbent increased linearly with increasing So, and the ultimate conversion increased linearly with increasing SgoM_1. The initial conversion rate and ultimate conversion of sorbent increased significantly with increasing relative humidity of the gas. Temperature and So(2) concentration had mild effects on the initial conversion rate and negligible effects on the ultimate conversion. (C) 2003 Elsevier Ltd.