In situ spectroscopic investigations of evaporated silver films as oxygen electrodes on yttria stabilized zirconia (YSZ) have been performed at 550 degreesC under UHV conditions. For this purpose a new three-electrode galvanic cell with oxygen gas on the reference side has been developed. The oxygen activity at the working electrode was changed by controlling the electrode potential between working and reference electrode. Ultraviolet photoelectron spectroscopy (UPS) was used to determine the work function change during pumping oxygen to the surface of the silver electrode. A change in valence band structure was observed during oxygen pumping. The work function increases and a new electronic state attributed to surface oxygen appears at a binding energy of 3.3 eV below the Fermi energy. With X-ray photoelectron spectroscopy (XPS) the chemical nature of this surface oxygen species was further investigated. A binding energy of the O Is core electrons was observed at 529 eV indicating the occurrence of subsurface oxygen. Scanning electron micrographs show characteristic changes in the electrode morphology after polarization.