Ammonia (NH3) is an attractive carbon-free fuel, yet its low reactivity presents many challenges for direct use in combustion applications. These combustion challenges could be resolved by mixing NH3 with more reactive fuels such as hydrogen (H2). To further contribute to NH3 and NH3/H2 kinetics—which arguably still requires much improvement—new experiments were conducted over a wide range of temperatures (1474–2307 K), near-atmospheric pressure, several NH3/O2 mixtures (equivalence ratios varying from 0.56 to 2.07), and near-stoichiometric NH3/H2/O2 mixtures with NH3:H2 ratios of 80:20 and 50:50. During these experiments, laser absorption diagnostics near 10.4 µm and 7.4 µm were simultaneously employed to measure NH3 and H2O time histories, respectively. Characteristic parameters, such as NH3 half-life time and H2O induction delay time, were extracted from the time-history profiles, and these parameters present stringent speciation targets for mechanism validation. After an assessment of most modern kinetics models, three, most accurate, mechanisms were compared against the experimental results. Only one model was able to partially reproduce the pure NH3 experiments, yet none of the models were capable of predicting the NH3/H2 experiments. Reaction pathway analysis showed that NH3 oxidation proceeds via forming NH2 then followed three different routes to form N2. Importantly, the models considered showed different levels of importance for each route. Sensitivity analysis showed that the NH3/H2 experiment is mostly sensitive to NH3+OH⇄NH2+H2O. Interestingly, this reaction showed no sensitivity for the NH3/O2 experiments. Overall, the models exhibited significantly slower reactivity than the NH3/H2 experiments, and the kinetics analysis showed that the start of this reactivity is governed by the levels of H-atoms in the early stages of the experiments. At these early stages of the experiments, propagation and branching reactions in the H2/O2 system are the main contributors to generating H radicals, along with the reaction NH3+H⇄NH2+H2 which proceeds in its reverse direction.