Removal Of SO2 from flue gas by the absorbent synthesized from coal fly ash and calcium oxide was studied under different reaction conditions to elucidate the effect of the coexistence of NO and O-2 in the flue gas. The presence Of O-2 and NO in the flue gas was found to be necessary to produce sulfate (SO42-) salts instead of sulfite (SO32-) salts as the final product of the desulfurization reaction. The roles of O-2 and NO were postulated as an oxidizing agent to oxidize SO2 to SO3, which then reacts with the absorbent. NO itself is not an oxidizing agent, but with the presence Of O-2, it can be oxidized to NO2 which acts as an oxidizing agent. It was also found that NO2 (from NO) is a better oxidizing agent compared to O-2 in oxidizing SO2 to SO3. (c) 2005 Elsevier Ltd. All rights reserved.