Solid oxide fuel cell (SOFC) electrolytes must be crystallographically and chemically stable in typical operating environments, while also possessing high ionic and low electronic conductivities. Cubic fluorite structured yttria-stabilized zirconia (YSZ), in which ion conduction is provided by oxide ion vacancies, is a proven oxide ion conductor fulfilling such requirements. In the present work, the microstructural and crystallographic defects of YSZ in the anode (Ni/YSZ), cathode (LSM/YSZ), and electrolyte of commercial SOFC cells were studied using Transmission Electron Microscopy (TEM). Comparisons were made between an as-received cell and a cell operated at 800 degrees C for 550 h using syngas as fuel. Examination of the YSZ electron diffraction pattern of the cathode reveals a typical cubic fluorite crystal structure. However, weak diffraction spots, where diffractions should be absent according to the structure factor of the cubic fluorite phase, are present in the patterns taken from the YSZ in the anode and the electrolyte. The appearance of kinematically forbidden spots in YSZ is unchanged after cell operation. Such weak diffraction spots can be interpreted as arising from a nanoscale (2-10 nm) irregularly-shaped tetragonal YSZ (t-YSZ) domain that is randomly distributed in the cubic YSZ (c-YSZ) matrix. However, larger t-YSZ domains with thicknesses of 5-10 nm and lengths of 30 nm are observed by the Ni/YSZ interfaces in cells operated at 800 degrees C in coal-derived syngas for 550 h. The layered t-YSZ domains are accompanied by the c-YSZ domains, implying the migration of Y along the Ni/YSZ interface in response to cell operation. (C) 2011 Elsevier B.V. All rights reserved.