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Abstract The development and refinement of NOx chemical kinetic mechanisms have been instrumental in understanding and reducing NOx formation. However, relatively little work has been performed with NOx species as the oxidizer, and such experiments can provide unique insights into NOx kinetics. Furthermore, speciation data can often provide useful information that complements global measurements such as ignition delay times in facilitating mechanism refinement. To provide such speciation data in the H₂‐NO₂system, H₂O measurements were performed using a fixed‐wavelength, direct absorption laser diagnostic near 1.39 µm behind reflected shock waves in fuel‐lean, near‐stoichiometric, and fuel‐rich mixtures of H₂and NO₂highly diluted in argon. Experiments were performed between 917 and 1782 K near atmospheric pressure. The H₂O profiles obtained herein are markedly different from those using O₂as the oxidizer obtained in a previous study. The GRI 3.0 mechanism was found to greatly underestimate the H₂O formation, whereas two modern mechanisms were found to predict the H₂O formation quite accurately except at colder temperatures for fuel‐rich conditions. Explanations for the differences between these mechanisms are given and discussed, with the conclusion that older mechanisms such as GRI 3.0 should not be used to model hydrocarbon/NOx combustion chemistry as they are lacking several key reactions and species, namely NO₃and HONO. The discrepancy between models and data at lower temperatures could not be reconciled even when modifying two of the most‐sensitive reaction rates. To the best of the authors’ knowledge, this study presents the first shock‐tube speciation study in the H₂‐NO₂system.