Auto-ignition properties of NH3/O-2 and NH3/H-2/O-2 mixtures have been studied in a rapid compression machine at pressures from 20 to 60 bar, temperatures from 950 to 1150 IC, and equivalence ratios from 0.5 to 2. The effect of the ammonia/hydrogen ratio in the fuel mixture has been also investigated. The experiments demonstrate that a higher H-2 mole fraction in the fuel mixture increases its reactivity, while the equivalence ratio shows different influence as follows. When the fuel mixture contains 20% H-2, the fuel-richer mixtures have shorter ignition delay times, while for mixtures containing 1% H-2 in fuel the equivalence ratio dependence is opposite. With 5% H-2 in fuel, the stoichiometric mixture presents the shortest ignition delay time. In mixtures without hydrogen, i.e., pure NH3, leaner mixtures show higher reactivity. In addition, numerical simulations were performed based on the literature mechanisms of Glarborg et al. (2018), Mathieu and Petersen (2015), and Klippenstein et al. (2011). While these models can predict well the ignition delay time of NH3/O-2 mixtures, none of the models can predict the behavior of NH3/H-2/O-2 mixtures satisfactorily. The predictions are most sensitive to the branching reactions NH2+NO and to the reaction H2NO + O-2 = HNO+ HO2. Hydrogen addition enriches the O/H radical pool consuming NH3 and NH2, but it has small effect on NOx emissions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.