Measurements of autoignition delay times of NH3 and NH3/H-2 mixtures in a rapid compression machine are reported at pressures from 20-75 bar and temperatures in the range 1040-1210 K. The equivalence ratio, using O-2/N-2 /Ar mixtures as oxidizer, varied for pure NH3 from 0.5 to 3.0; NH3/H-2 mixtures with H-2 fraction between 0 and 10% were examined at equivalence ratios 0.5 and 1.0. In contrast to many hydrocarbon fuels, the results indicate that, for the conditions studied, autoignition of NH3 becomes slower with increasing equivalence ratio. Hydrogen is seen to have a strong ignition-enhancing effect on NH3. The experimental data, which show similar trends to those observed previously by He et al. (2019) [28], were used to evaluate four NH3 oxidation mechanisms: a new version of the mechanism described by Glarborg et al. (2018) , with an updated rate constant for the formation of hydrazine, NH2 + NH2 (+M) = N2H4 (+M), and the literature mechanisms from Klippenstein et al. (2011) [30], Mathieu and Petersen (2015) [25], and Shrestha et al. (2018) [31]. In general, the mechanism from this study has the best performance, yielding satisfactory prediction of ignition delay times both of pure NH3 and NH3/H-2 mixtures at high pressures (40-60 bar). Kinetic analysis based on present mechanism indicates that the ignition enhancing effect of H-2 on NH3 is closely related to the formation and decomposition of H2O2 ; even modest hydrogen addition changes the identity of the major reactions from those involving NHx radicals to those that dominate the H-2/O-2 mechanism. Flux analysis shows that the oxidation path of NH3 is not influenced by H-2 addition. We also indicate the methodological importance of using a non-reactive mixture having the same heat capacity as the reactive mixture for determining the non-reactive volume trace for simulation purposes, as well as that of limiting the variation in temperature after compression, by limiting the uncertainty in the experimentally determined quantities that characterize the state of the mixture. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.