The selective catalytic reduction (SCR) of nitrogen oxides (NO,) by a combination of dielectric barrier discharge plasma and a monolithic V2O5-WO3/TiO2 catalyst using ammonia as a reducing agent was studied in synthetic gas mixtures at temperatures between 100 and 250 degreesC. The mixtures were similar to diesel exhaust gases. For gas mixtures (NOx,0 = NH3,0 = 590 ppm) in which 95% of the nitrogen oxides were in the form of NO, the removal of NOx below 140 degreesC without plasma treatment was negligible. Treating the gas mixture with dielectric barrier discharges before the catalytic conversion, about 70% of the NO2 was reduced at temperatures as low as 100 degreesC. By plasma treatment 170 ppm of the NO was converted, 110 ppm by oxidation to NO2 and 60 ppm by reduction with products of NH3. Due to the coexistence of NO and NO2 on the catalyst, the selective catalytic reduction was enhanced. Similar effects were observed for the selective catalytic reduction in gas mixtures containing equal amounts of NO and NO2 without plasma treatment. Relative reaction rates for different NOx-reducing reactions over the catalyst are evaluated using a macroscopic model.