The application of Yttria-stabilized Zirconia (YSZ) as solid electrolyte in high-temperature solid oxide fuel cells (SOFC) is well established. However, the strong decrease of the ionic conductivity in 8.5 mol% Y2O3-doped ZrO2 at high temperature has not yet been clarified completely. To contribute to the understanding of the degradation process, transmission electron microscopy (TEM) was applied to analyze the microstructure in YSZ electrolyte substrates in as-sintered and aged material. Selected area electron diffraction and conventional TEM imaging were performed to investigate the evolution of different phases and phase transitions in YSZ. Grain boundary charging and the possible formation of a glassy phase at grain boundaries after aging were investigated using transmission electron holography and high-resolution TEM. The ionic conductivity was characterized by dc-conductivity measurements and impedance spectroscopy. (c) 2006 Elsevier B.V. All rights reserved.