The effect of partial pressures of the reactants in the Thermal DeNOx process has been investigated in flow reactor experiments. The experiments were performed at atmospheric pressure for temperatures ranging from 923 to 1373 K. Initial concentrations were varied for NH3NO (400200, 1000500, 28001400ppm) and O2 (0–50%). the data confirm earlier observations that in the temperature range covered, presence of O2 is required in order for NO to be reduced by NH3. As the initial O2 concentration is increased, the lower boundary for the process is shifted towards lower temperatures. The temperature range for NO reduction is widened, but the NO reduction potential decreases. At high oxygen concentrations the maximum NOx reduction is below 40%. Under these conditions, significant amounts of NO2 and N2O are formed. Two mechanisms for N2O formation in Thermal DeNOx have been identified. One is active at higher temperatures and low O2 concentrations, while the other, which presumably involves NO2 as a precursor, is dominant at lower temperatures and high O2 levels. The implications of the results for application of Thermal DeNOx in high pressure systems such as pressurized fluidized bed combustion is discussed. Comparisons of the experimental data with recent chemical kinetic models indicate that the detailed chemistry of the Thermal DeNOx system is not completely understood.