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The segregation behavior in 3 and 10 mol % polycrystalline yttria stabilized zirconia ͑YSZ͒, calcined at temperatures ranging from 300 to 1600°C, is characterized using low-energy ion scattering ͑LEIS͒. In order to be able to separate the Y and Zr LEIS signals, YSZ samples have been prepared using isotopically enriched 94 ZrO 2 instead of natural zirconia. The samples are made via a special precipitation method at a low temperature. The segregation to the outermost surface layer is dominated by impurities. The increased impurity levels are restricted to this first layer, which underlines the importance of the use of LEIS for this study. For temperatures of 1000°C and higher, the oxides of the impurities Na, Si, and Ca even cover the surface completely. The performance of a device like the solid oxide fuel cell which has an YSZ electrolyte and a working temperature around 1000°C, will, therefore, be strongly hampered by these impurities. The reduction of impurities, to prevent accumulation at the surface, will only be effective if the total impurity bulk concentration can be reduced below the 10 ppm level. Due to the presence of the impurities, yttria cannot accumulate in the outermost layer. It does so, in contrast to the general belief, in the subsurface layer and to much higher concentrations than the values reported previously. The difference in the interfacial free energies of Y 2 O 3 and ZrO 2 is determined to be Ϫ21Ϯ3 kJ/mol.