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We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe 2 O 3 . Passing CO and H 2 O, but no O 2 , over Au-γ-Fe 2 O 3 at 25 °C, we observe significant CO 2 production, inconsistent with LH mechanistic hypotheses. Experiments with H 2 18 O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe 2 O 3 . Guided by density functional theory, we instead postulate a water-promoted Mars-van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO 2 production in the absence of O 2 and with CO oxidation in the presence of H 2 18 O and 16 O 2 . In contrast, for Au-TiO 2 , our data is consistent with previous LH mechanistic hypotheses., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)