The effects of the addition of NaOH on the reactivities toward SO2 of iron blast furnace slag/hydrated lime (BFS/HL) sorbents were studied without and with the presence of flue gas components, CO2, O-2, and NOx, Under the conditions prevailing in the bag filters of dry and semidry flue gas desulfurization processes. The reactivities of sorbents were markedly enhanced by the addition of NaOH when CO2, O-2, and NOx were not present in the gas mixture. The enhancement effect mainly resulted from the deliquescent property of NaOH. With the presence of CO2, O-2, and NOx at their typical concentrations in the flue gas, the sorbent reactivity was greatly increased due to the oxidation of bisulfite and sulfite ions to Sulfate ions as well as the formation of deliquescent salts, calcium nitrite and nitrate, being enhanced by the presence of NO2 during the sulfation of a BFS/HL sorbent. The addition of NaOH in this case promoted the formation of sulfates but became less important in raising the sorbent reactivities for sorbents except BFS. Without the presence of CO2/O-2/NOx, the BFS/HL sorbents with ratios <= 70/30 had higher reactivities than HL, but with the presence of CO2/O-2/NOx, HL had the highest Value of SO2 capture among the BFS/HL and BFS/HL/NaOH sorbents tested. Placing the SO2 removal unit before the NOx, removal unit in practical operations call take advantage of the enhancement effect of CO2/O-2/NOx on increasing the Ca utilization and SO2 capture of the sorbent.