In this paper, the nitrogen chemistry in an ammonia (NH3) doped flameless jet is investigated using a kinetic reactor network model. The reactor network model is used to explain the main differences in ammonia chemistry for methane (CH4) containing fuels and methane-free fuels. The chemical pathways of nitrogen oxides (NOx) formation and destruction are identified using rate-of-production analysis. The results show that in the case of natural gas, ammonia reacts relatively late at fuel lean condition leading to high NOx emissions. In the pre-ignition zone, the ammonia chemistry is blocked due to the absence of free radicals which are consumed by methane-methyl radical (CH3) conversion. In the case of methane-free gas, the ammonia reacted very rapidly and complete decomposition was reached in the fuel rich region of the jet. In this case the necessary radicals for the ammonia conversion are generated from hydrogen (H-2) oxidation. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.