Pt 3 Ni(111)i st he most efficient catalystf or the oxygen reduction reaction. Herein, we investigated the oxidation process of Pt 3 Ni(111)b yt ime-resolved X-ray photoemissions pectroscopy experiments with synchrotronr adiation. For O 2 exposure at room temperature, two well-distinct Os pecies are present with as tickingc oefficient of 0.23-0.24 in the temperature range of interest for fuel-cell technology.W es how that the Pt skin of Pt 3 Ni(111)i ss electively replaced by Ni 2 O 3 and NiO surface-skinl ayers upon heatingi na nO 2 environment at 700 and 900 K, respectively.B yd osingw ith Oa toms, the formation of PtO is possible at 293 K, whereas Ni 2 O 3 is formed at 353 K.Proton-exchange membrane fuel cells (PEM-FCs) are ap romising ecofriendly alternative to combustion enginesf or automotive applications. In PEM-FCs, both anode and cathode reactions are catalyzed by Pt-or Pt-based electrocatalysts.[1] The hydrogen oxidation reaction on Pt anodes is intrinsically fast and requires very little Pt. By contrast, the cathode oxygen reduction reaction (ORR) is as low reactiont hat consumes approximately 90 %o ft he total Pt content in PEM-FCs.[2] Pt is ap recious and expensive metal, and consequently,i ts use limits the large-scale commercialization of PEM-FCs. The current Pt loading in the most-advanced fuel-cellv ehicles using state-of-theart Pt-based catalysts is approximately four-to eightfold higher than the targete stablishedf or mass productiono ff uelcell vehicles.H ence,l owering the Pt loading at the cathode is the most criticalm ission for the advancement of PEM-FC technology.[3] In particular, significant effort is required for comprehension of the ORR on Pt-and Pt-based electrocatalyst surfaces. The search for novel Pt-based electrocatalysts with enhanced ORR activity is seemingly the mostp rofitable pathway. Specifically,e lectrodes composed of Pt and another 3d transition metal, such as Co, [4] Fe, [5] or Ni, [4,6,7] exhibit significantly higher activity for the ORR than pure Pt. Stamenković and coworkers [6] have demonstrated that the Pt 3 Ni(111)s urface is 10 times more active for the ORR than the corresponding Pt(111) surfacea nd 90-foldm ore activet han the current state-of-theart Pt/C catalysts for PEM-FCs. The Pt 3 Ni(111)s urface has ap eculiar arrangemento fs urfacea toms in the near-surfacer egion. Under operating conditions relevant to fuel cells, the outermost layer of the Pt 3 Ni(111)s urfacei se ntirely composed of Pt atoms (Pt skin [8,9] ), whereas the second layer is Ni rich (52 %) [10] and the third layer is again Pt rich (87 %).[10] It has also been demonstrated by both experimentalists [11] and theoreticians [12] that in an oxygen environment Ni atoms segregatet oward the surfaceo fP t 3 Ni(111)t of orm aN iO skin. The presence of Ni 2 O 3 in the surface composition of Pt 3 Ni(111), modified by oxygen exposure, has also been reported. [13] However,t ot ailor more effective Pt 3 Ni-based electrodes, it is mandatory to understand the microscopic mechanism of oxygen interaction wit...