The surfaces of noble metal single crystals such as Pt(111) have served as successful model systems for heterogeneous catalysts. To account for the fact that supported Pt nanoparticles of technological catalysts typically exhibit different low Millerindex facets to the reactive gases, single-crystal studies were most frequently carried out for Pt(111), Pt(110) and Pt(100) terminations. Catalytic reaction properties were, for example, determined by temperature-programmed methods or molecular beam studies (both relying on mass spectroscopic product analysis) or, in case of atmospheric pressure conditions, by gas chromatography. In order to evaluate and compare the catalytic properties of the different crystallographic orientations, a relatively large parameter space of reactant gas pressure and reaction temperature must be investigated. Since this represents a significant body of work, there is hardly any catalytic system with a complete set of reported reactivity data. We present an alternative approach that allowed us to determine catalytic properties of different crystallographic terminations in a more efficient way, at least for specific surface reactions. Photoemission electron microscopy (PEEM) was employed to study the CO oxidation reaction on polycrystalline Pt foil, consisting of micrometer-sized domains of (100), (110) and (111) terminations. PEEM imaging was performed in situ, that is, during the reaction of CO and oxygen to give CO2, and the analysis of the local photoemission intensity of selected domains on the Pt foil enabled us to obtain locally resolved kinetic information for (100), (110) and (111) surfaces. It is important to note that at a given time the reactant gas pressure and temperature were identical for all terminations, allowing a direct simultaneous comparison of their catalytic properties. The kinetic behavior of the (100), (110) and (111) surfaces is described by “kinetic phase diagrams” (also known as bifurcation diagrams; for details see the Supporting Information), representing the domain-specific catalytic properties. It is shown that the (111)-, (100)and (110)-oriented grains behaved almost identical to the corresponding single crystals and that the superposition of the weighted contributions of the individual domains (as measured by PEEM) reproduced the global kinetics of CO oxidation on Pt foil (as measured by MS). Under the applied experimental conditions, the (100), (110) and (111) domains behaved independently to a large extent, indicating that diffusion and gas-phase coupling between the different facets were not sufficient to synchronize their kinetic transitions. The microscopic observation of reaction fronts that were confined within the domain boundaries corroborated the insufficient coupling. Implications of the current results on CO oxidation on supported Pt nanoparticles of technological catalysts are also discussed. Kinetic phase transitions on the Pt(111) single-crystal surface have been previously observed by PEEM (by averaging the whole image intensity, since the homogeneity of such a surface did not ask for spatially-resolved analysis) and PEEM images of CO oxidation on Pt foil have been reported (but without analysis of phase transitions). However, the present contribution shows for the first time how local kinetic phase diagrams of a catalytic reaction can be obtained for individual differently oriented domains of a polycrystalline material. The oxidation of CO on platinum surfaces is a seemingly simple reaction, but it exhibits several complex phenomena such as hysteresis (bistability), oscillations, dissipative structures and chaotic behaviour. Herein, we have concentrated on bistability and deliberately did not examine the parameter space (higher temperature and pressures) of oscillations which were extensively studied before. To illustrate the kinetic behavior of Pt foil, Figure 1 (right inset) shows the global (overall) reaction kinetics measured by mass spectroscopy. At a reaction temperature of 417 K, the Pt foil was exposed to a constant partial pressure of oxygen (1.3 10 5 mbar) while the CO partial pressure (pCO) was cycled and the CO2 production rate (RCO2 ) was monitored by mass spectroscopy. Upon exposing the oxygen-covered Pt surface to increasing CO pressure, RCO2 increased (high activity) until a kinetic transition point at pressure tA was reached. When pCO exceeded tA, the Pt surface switched from oxygen-covered to CO-covered, marked by the loss of catalytic activity due to CO self-poisoning (oxygen cannot adsorb on the CO-covered surface). When pCO was decreased, the catalyst remained in the poisoned (low reactivity) steady state until a second transition pressure tB was approached. Accordingly, at pCO tB<pCO<pCO , the catalyst can adopt one of two steady states, depending on the prehistory. The resulting hysteresis is characteristic for a bistable reaction behavior that originates from the asymmetric inhibition of dissociative oxygen adsorption by CO:oxygen needs two neighboring adsorption sites per molecule for dissociation and can not adsorb on a densely packed CO-covered [a] Prof. Dr. Y. Suchorski, C. Spiel, D. Vogel, Dr. W. Drachsel, Prof. Dr. G. Rupprechter Institute of Materials Chemistry Vienna University of Technology Veterin rplatz 1, 1210 Vienna (Austria) Fax: (+43)1-58801-16599 E-mail : grupp@imc.tuwien.ac.at [b] D. Vogel, Prof. Dr. R. Schlcgl Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg, 4–6, 14195 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000599.