Yttria-doped zirconia (YDZ) has been used as electrolyte for solid oxide fuel cells (SOFCs) for many years. Nevertheless, fundamental questions regarding the thermal stability and thus the degradation of ionic conductivity of YDZ are controversially discussed in literature. In this study, thick-film and sol-gel prepared thin-film YDZ electrolytes with yttria concentrations in the range of 7.3-10 mol% were investigated by transmission electron microscopy (TEM) with emphasis on microstructural and chemical changes during high-temperature operation. In general, nanoscaled regions of metastable...