Electrochemical polarization of porous metal electrodes on solid electrolytes often leads to an increase of their catalytic activity in heterogeneous reactions. Thin microstructured Pt films on single crystalline YSZ (yttria stabilized zirconia) are investigated as structurally and geometrically well-defined model systems in order to understand the origin of this effect. Photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) have been applied as spatially resolving methods in situ to study the processes in the vicinity of the three-phase boundary (tpb). Measurements with SPEM show that atomic oxygen is created under anodic polarization and covers the Pt film homogeneously. A specific spillover species is not found. rather the formation of atomic oxygen is detected, which has the same: Ols binding energy as chemisorbed oxygen from the gas phase.