Methyl orange (MO) azo dye served as model organic pollutant to investigate the role of reactive nitrogen species (RNS) in non-thermal plasma (NTP) induced water treatments. The results of experiments in which MO aqueous solutions were directly exposed to N-2-NTP are compared with those of control experiments in which MO was allowed to react with nitrite, nitrate and hydrogen peroxide, which are species formed in water exposed to N-2-NTP. Treatment of MO was also performed in PAW, Plasma Activated Water, that is water previously exposed to N-2-NTP. Both direct N-2-NTP and N-2-PAW treatments induced the rapid decay of MO. No appreciable reaction was instead observed when MO was treated with NO3- and H2O2 either under acidic or neutral pH. In contrast, in acidic solutions MO decayed rapidly when treated with NO2- and with a combination of NO2- and H2O2. Thorough product analysis was carried out by HPLC coupled with UV-vis and ESI-MS/MS detectors. In all experiments in which MO reaction was observed, the major primary product was a derivative nitro-substituted at the ortho position with respect to the N,N-dimethylamino group of MO. The reactions of RNS are discussed and a mechanism for the observed nitration products is proposed. (C) 2015 Elsevier B.V. All rights reserved.