Catalytic partial oxidation of methane to synthesis gas (CPOM) over yttrium-stabilized zirconia (YSZ) was studied within a wide temperature window (500¿1100 °C). The catalysts were characterized by X-ray fluorescence (XRF) and low-energy ion scattering (LEIS). The influence of calcination temperature, Y2O3 content, and especially impurities such as CaO, TiO2, and Na2O on catalytic performance were investigated. Creation of active sites by doping with Y2O3 improves the catalytic performance of ZrO2 significantly. The surface composition rather than the bulk composition determines the catalytic performance of the catalysts in CPOM. As long as YSZ catalyst is not contaminated, the composition of the outermost surface of calcined YSZ is independent of both the concentration of Y2O3 in the bulk and calcination temperature; the surface always contains 12 ± 2 mol% Y2O3 due to segregation of Y2O3. Calcination at higher temperatures creates more active sites per square meter, while the catalyst loses surface area via sintering. The same sintering treatment causes the activity of YSZ containing traces of (earth) alkali oxides to collapse. The effect is probably due to segregation of the impurities to the surface, which either blocks the active surface of YSZ catalyst or forms new phases with different catalytic properties. However, it cannot be ruled out that enhanced segregation of Y2O3 contributes to this effect as well. Heterogeneous reactions occur concurrently with homogeneous reactions at temperatures above 950 °C during CPOM over YSZ. At such high temperatures, CPOM, steam- and CO2 reforming, and reverse water¿gas shift occur in competition during CPOM. These reforming reactions of methane result in a significant increase in synthesis gas selectivity, although the catalyst activity is still too low to reach thermodynamic equilibrium.