This study focuses on the clarification of the accelerated degradation of the ionic conductivity of Ni-containing 8.5 mol% Y2O3-doped ZrO2 (8YDZ) under reducing atmosphere, a transition that proceeds 50 times faster in comparison to pure 8YDZ. A comparative study of high-performance anodes and NiO/YDZ thin-film specimens (NiO thin film on thick 8YDZ and 10YDZ electrolytes) has been carried out to characterize the microstructural and chemical evolution of the YDZ under operating conditions (reducing atmosphere). The extremely deep indiffusion of Ni (>100 mu m) into microcrystalline YDZ was revealed after sintering at 1400 degrees C for 5 h (air). Hence, such Ni-containing 8YDZ is expected to form whenever NiO and 8YDZ are co-sintered at high temperature, e.g., during half-cell (anode with thin-film electrolyte) fabrication. Moreover, the spinodal decomposition of the Ni-containing 8YDZ on the short time scale was identified and studied by conventional and analytical transmission electron microscopy (TEM). This comprises both the microstructural coarsening as well as the strong chemical decomposition on the nanoscale. Since the decomposition rate is directly related to the mobilities of the host cations these have to be enhanced by the dissolved Ni. The explanation therefore is based on a localized strain field that is expected to arise in the vicinity of individual Ni species due to electron capture under reducing atmosphere. It is proposed that the coulomb interactions are weakened (due to the expansion of the YDZ lattice) while lattice vibrations are enhanced leading to lower potential barriers for ion hopping and thus to higher cation mobilities. (C) 2012 Elsevier B.V. All rights reserved.