A two-step process capable of removing NOx and SO2 simultaneously was proposed, which was made up of an ozonizing chamber and an absorber containing a reducing agent solution. Nitrogen oxides (NO plus NO2) in most practical exhaust gases consist chiefly of NO. The injection of ozone into the exhaust gas gives rise to a rapid oxidation of NO to NO2. Compared to NO, NO2 has relatively high solubility in water, and it can readily be reduced to N-2 when the NO2-rich exhaust gas is brought into contact with the reducing agent solution. Sodium sulfide (Na2S) used as the reducing agent in this study can also remove SO2, effectively. As the exhaust gas passed through the ozonizing chamber and the absorber sequentially, NOx removal efficiency of about 95% and SO2 removal efficiency of 100% were obtained. The formation of H2S from sodium sulfide could be suppressed by using a basic reagent, together with the reducing agent. The rate of depletion of the reducing agent during the treatment of the exhaust gas was much faster than expected by reaction stoichiometry, obviously due to the oxygen in the exhaust gas. The amount of sodium sulfide required was found to be about four times the amount of NOx and SO2 removed. (C) 2005 Elsevier B.V. All rights reserved.