It has been considered to apply Gd0.1Ce0.9O1-x (GDC) to low temperature solid oxide fuel cell (SOFC) electrolytes at a low temperature. However, ceria cannot be efficiently used with hydrogen and wet hydrocarbon fuels because its structure change (Ce4+-> Ce3+) causes a decline of mechanical strength under a reducing atmosphere. In this work, the structural stability test of the GDC electrolyte was investigated under H-2 and dry CH4 atmospheres depending on the fuels at 650 degrees C. This study clearly confirmed from the scanning electron microscopy, X-ray photoelectron spectroscopy, and mechanical strength data that many more Ce4+ ions transform into Ce3+ ions under the reducing (H-2) atmosphere [J. Alloys Compd., 408-412, 628 (2006); Solid State Sci., 7, 539 (2005)]. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3465324] All rights reserved.