NaOH was added to iron blast furnace slag (BFS)/hydrated lime (HL) slurries to prepare sorbents for semidry and dry flue gas desulfurization (FGD) processes. The addition of NaOH greatly enhanced the sorbent reactivities toward SO2 because NaOH is deliquescent and its presence increased the amount of water collected by the sorbent. In the case of sorbents prepared without NaOH addition, both the Ca utilization and the SO2 capture increased with increasing slurrying temperature and varied with the BFS/HL weight ratio; however, in the case of sorbents prepared with NaOH addition, their Ca utilization increased to a level that was nearly independent of the slurrying temperature and the BFS/HL weight ratio. NaOH addition can raise the level of sorbent reactivity and reduce the sorbent cost in a semidry or dry FGD process.