In this paper, the NO-to-NO2 conversion in various gaseous mixtures is experimentally investigated. Streamer coronas are produced with a dc-superimposed high-frequency ac power supply (10-60 kHz). According to NOx removal experiments in N-2 + NOx and N-2 + O-2 + NOx gaseous mixtures, it is supposed that the reverse reaction NO2 + O --> NO + O-2 may not only limit NO(2)Z production in N-2 + NOx mixtures, but also increase the energy cost for NO removal. Oxygen could significantly suppress reduction reactions and enhance oxidation processes. The reduction reactions, such as N + NO --> N-2 + O, induce negligible NO removal provided the O-2 concentration is larger than 3.6%. With adding H2O into the reactor, the produced NO2 per unit removed NO can be significantly reduced due to NO2 oxidation. NH3 injection could also significantly decrease the produced NO2 via NH and NH2- related reduction reactions Almost 100% of NO2 can be removed in gaseous mixtures of N-2 + O-2 + H2O + NO2 with negligible NO production.