NaOH was added to iron blast furnace slag (BFS)/hydrated lime (HL) slurries to prepare the sorbents for semidry and dry flue gas desulfurization (FGD) processes. For sorbents prepared without NaOH, both the Ca utilization and the SO 2 capture increased with increasing slurrying temperature and varied with the BFS/HL weight ratio. With NaOH addition, the sorbents contained much less high surface area products of pozzolanic reaction. Their reactivities were insensitive to the slurrying temperature and greatly enhanced when the NaOH/(BFS + HL) ratio was about 10/100. NaOH is deliquescent, and its presence increases the amount of water collected by a sorbent, which is required for the sulfation of a sorbent at low temperatures. The use of NaOH addition at or below the optimal ratio can raise the sorbent reactivity and reduce the cost of sorbent for a semidry or dry FGD process.