We report here on the oxidation of monolayer FeO islands on Pt(1 1 1) into the highly-reactive FeO_2-x phase by high-temperature oxidation under Ultra High Vacuum (UHV) conditions. The chemical composition and characteristic phonon modes of both FeO and FeO_2-x films were investigated by X-ray photoemission Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS). The reactivity and thermal stability of the “oxygen-rich” FeO_2-x phase were studied with respect to the CO oxidation reaction at 450 K and to annealing in UHV at 563 K, respectively. By performing repeated oxidation/reduction cycles, we have identified the vibrational mode at 76 meV as the unique signature of the FeO_2-x phase and confirmed the already reported ability of CO to reduce such oxide at 450 K. The latter process is not fully reversible, suggesting a modification of the active sites upon CO adsorption.