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The fundamental characteristics of flue gas cleaning for NOx using a wet-type plasma reactor are evaluated, with attention being laid on concentrations of nitrate and ammonium ions in the liquid. The wet-type plasma reactor of the wire-cylinder configuration driven by a square-wave high voltage was used. A thin liquid film was maintained on the inner wall of the reactor. In this reactor, a discharge plasma oxidizes NO to NO2, and NO2 is dissolved into the liquid as NO3-. Continuous absorption of nitrogen oxides induces saturation and acidification of the liquid, therefore inhibiting further absorption. The effect of the addition of ammonium ions (NH4+) into the liquid film on the enhancement of NOx removal has been experimentally confirmed. In that case, the addition of NH4+ was effective for NO oxidation, as well as NOx absorption into the liquid, resulting in the increase of NO2- and NO3- concentrations in the liquid. With the presence of Fe2+ ions in the liquid exposed to the discharge plasma, a reduction of NO3- to NH4+ takes place. The reduction of nitrate ions to ammonia raises the solution pH and ensures continuous NOx absorption. These results suggest the possibility of the continuous operation of the wet-type plasma reactor for NOx removal without excess acidification of the absorbing water. The process that is responsible for the reduction of nitrate ions into ammonium ions is of an electrochemical nature.