AbstractOperando X‐ray absorption spectroscopy identified that the concentration of Fe²⁺ species in the working state‐of‐the‐art Pt−FeO_x catalysts quantitatively correlates to their preferential carbon monoxide oxidation steady‐state reaction rate at ambient temperature. Deactivation of such catalysts with time on stream originates from irreversible oxidation of active Fe²⁺ sites. The active Fe²⁺ species are presumably Fe⁺²O⁻² clusters in contact with platinum nanoparticles; they coexist with spectator trivalent oxidic iron (Fe³⁺) and metallic iron (Fe⁰) partially alloyed with platinum. The concentration of active sites and, therefore, the catalyst activity strongly depends on the pretreatment conditions. Fe²⁺ is the resting state of the active sites in the preferential carbon monoxide oxidation cycle.