Processes of nitrogen oxides decomposition in non-equilibrium plasma generated in electrical discharges are accompanied by a re-synthesis of nitrogen oxides. Fathoming of the effect of HV feeder electrical parameters and of gas composition on the NO re-synthesis process can help to inhibit the process and to obtain a higher degree of the NO decomposition and more efficient utilisation of electrical energy that feeds a reactor. Tests have been performed in a flat reactor with a quartz dielectric (discharge gap of 1 mm) fed with alternate voltage (50-4500 Hz) of 10.0 kV. Feeding gas consisted of 9 to 95% of O₂, 0-8000 ppm of H₂O with the remaining part being nitrogen. An analysis of gas composition at the reactor outlet has been performed with a mass spectrometer and it has shown that among the synthesis products aside with ozone there also are N₂O, NO, NO₂ and N₂O₅ there. Along with a growing feeding voltage value the total NOX concentration increases. The highest total NOX concentration values have been obtained at the 38% oxygen content in the mixture. Further increase of the oxygen content in the mixture causes a decrease of the N₂O concentration. An increase of the vapour content causes a decrease of the N₂O and NO₂ concentrations. The obtained results indicate that in the process of NO_X removal from flue gases it is possible to inhibit the NO_X re-synthesis by an adequate selection of the HV feeder parameters and the composition of a gas that is subdued to the discharge action.