In the context of a hypothetic hydrogen based economy, ammonia has been suggested as a potential alternative to liquid fossil fuels for spark ignition (SI) engines. However, its use requires a detail understanding of its combustion characteristics, particularly the comprehension of the unavoidable thermal and fuel nitrogen oxides formation. This study presents the elaboration of an improved ammonia combustion mechanism validated for the flame structure prediction of ammonia, hydrogen, oxygen, argon flames investigated at several low pressures and for various conditions of equivalence ratio and of initial hydrogen content. The improved kinetic model is then reduced in order to be used in a complete SI engine numerical simulation. The comparison of the predictions of these mechanisms for the same ammonia flame structure is presented. The reduced mechanism contains 80 elementary reactions and 19 chemical species and allows a better understanding of the complete nitrogen oxides formation pathways. (c) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.