We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe2O3. Passing CO and H2O, but no O-2, over Au-gamma-Fe2O3 at 25 degrees C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with (H2O)-O-18 further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-gamma-Fe2O3. 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 CO2 production in the absence of O-2 and with CO oxidation in the presence of (H2O)-O-18 and O-16(2). In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.