<p>The direct conversion of methane to methanol (DCMM) under continuous flow and atmospheric pressure offers notable environmental benefits and industrial promise, but remains a long-standing challenge due to the difficulty of activating CH₄while avoiding over-oxidation of methanol. Here, we demonstrate that pure ceria (CeO₂), without any metal promoters, enables gas-phase DCMM with up to 80 % selectivity at 300–350 °C, upon optimization of the H₂O/O₂ ratio. At 550 °C, methanol and formaldehyde are formed at rates of 24 and 36 μmol g^-1 h^-1, respectively-both dropping below 1 μmol g^-1 h^-1 in the absence of O₂. Ex situ transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy confirm that CeO₂ maintains structural integrity and resists carbon deposition during reaction. Combining kinetic studies, steady-state <em>in-situ </em>diffuse reflectance infrared Fourier transform spectroscopy (<em>in-situ</em> DRIFTS), and density functional theory (DFT) reveals that hydroxyl groups (OH), generated from water dissociation, play a multifaceted role: they facilitate C–H bond activation, promote methoxy formation, and enhance methanol desorption. <em>In-situ</em> ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) directly reveals the evolution of surface intermediates and shows that co-feeding O₂ and H₂O suppresses CH₃O and CH_x accumulation while boosting methanol yield—indicating a rapid intermediate turnover as key to sustained activity. AP-XPS O 1s spectra further highlight that O₂ promotes H₂O dissociation, regenerating reactive OH groups and maintaining performance at elevated temperature. These findings offer molecular-level insights into how water and oxygen cooperatively tune reactivity, enabling efficient methane-to-methanol conversion on a metal-free oxide catalyst.</p>