Nitrogen oxides (NO + NO₂) in most practical exhaust gases is mainly composed of NO. The injection of ozone, produced by dielectric barrier discharge, into the exhaust gas gives rise to rapid oxidation of NO into NO₂. Once NO is converted into NO₂, it can readily be reduced to N₂ by a reducing agent in the next step. Sodium sulfide (Na₂S) used as the reducing agent can also remove SO₂ effectively, which makes it possible to treat NO_x and SO₂ simultaneously. The present two-step process through an ozonizing chamber and an absorber containing a reducing agent solution was able to remove about 95% of the NO_x and 100% of the SO₂, initially contained in the simulated exhaust gas. The formation of H₂S from sodium sulfide was able to be suppressed by a basic reagent. The use of NaOH and CaCO₃, together with the reducing agent not only prevented H₂S from forming, but also cut down on the consumption of the reducing agent.