We have used X-ray photoelectron spectroscopy and polarization-resolved 0 K-edge X-ray absorption spectroscopy to investigate the reactivity of various oxygen covered Pt(111) surfaces, which emerge under high temperature and pressure conditions, toward CO. We find that the reactivity of the O/Pt(111) system decreases monotonically with increasing oxygen coverage. Of the three surface oxygen phases, viz., chemisorbed oxygen (O-ad), a PtO-like surface oxide, and alpha-PtO2 trilayers, O-ad exhibits the highest reactivity toward CO, whereas alpha-PtO2 trilayers exhibit the lowest. Pt(111) surfaces fully terminated by alpha-PtO2 trilayers are inert to CO. Here it is proposed that the reactive phase is either O-ad or PtO-like surface oxide phase on bare non-CO poisoned Pt regions with PtO2 as majority spectator species.