The oxidative potential of a non-thermal plasma (NTP) in engine off-gases with excess oxygen results in an effective conversion of NO to NO2 that can be converted synergistically to molecular nitrogen with appropriate catalysts by selective catalytic reduction of NO2 with hydrocarbons (HC-SCR). The hydrocarbon added has two essential functions: first, it assists the gas-phase oxidation of NO to NO2 by the electric discharge in excess oxygen and, secondly, it reacts with NO2 in the hydrocarbon SCR. Besides CO2 and CO, significant amounts of formaldehyde and acetaldehyde are formed in the plasma-initiated gas-phase reaction. These and other by-products are involved together with the remaining propene in the subsequent catalytic reaction. The combination of SCR and cold plasma enhances the overall reaction and allows an effective removal of NOx at relatively low temperatures. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in this reaction. With an energy effort of ca. 30 eV per NO-molecule, a temperature of 300 degreesC and a space velocity of 20,000 h(-1) at the catalyst, it is possible to reduce 500 ppm NO in excess oxygen by more than half. The synergistic combination of NTP and HC-SCR has been verified under real conditions with exhaust gas from a diesel engine.