<p>In this study, Mn_xCe_(1−x) mixed oxides (x = 0, 0.25, 0.50, 0.75, and 1.00) were synthesized using a straightforward redox method, and the effect of the Mn–Ce composition on the total oxidation of carbon monoxide (CO) was examined. Under standard reaction conditions of 1% CO and 4% O₂, the light-off temperatures (T_50%), which indicate the temperature at which 50% of the CO conversion occurs, of Mn–Ce mixed oxide increased in the sequence: 0.25 &lt; 0.50 &lt; 0.75 &lt; 1.00 (MnO₂) &lt; 0 (CeO₂). Although Mn_0.25Ce_0.75 stands out for its superior catalytic performance, this efficiency notably diminishes when CO₂ and H₂O vapors are introduced into the reactants. This decline is attributed to the competitive adsorption of CO₂ and H₂O on the active sites, which impedes the oxidation of CO. Various techniques have been employed to understand the physicochemical properties of Mn–Ce mixed oxides. The analysis revealed that incorporating an appropriate amount of Mn into Ce yielded a substantial quantity of labile oxygen species, enhancing the reducibility of the system, thereby improving its catalytic performance. Although improving H₂O resistance is essential, Mn–Ce mixed oxides present a promising alternative to noble-metal-based oxidation catalysts.</p> Graphical Abstract <p></p>