The Young's modulus and the shear modulus of Ce0.9Gd0.1O2 - delta (GDC) were evaluated by using a resonance method in the temperature range from room temperature to 1273 K and in the p(O-2) range from 10(-24) to bar. It was found that the elastic moduli of GDC under oxidizing conditions gradually decreased with increasing temperature in a constant atmosphere and was almost independent of p(O-2) at a constant temperature. However, under reducing conditions, the elastic moduli apparently decreased as p(O-2) decreased even at a constant temperature, and such a decrease became more considerable under lower p(O-2) conditions. The variation in the elastic moduli of GDC with p(O-2) seemed to reflect the variation in the oxygen nonstoichiometry delta. It was demonstrated that the elastic moduli are affected not only by the change in the interatomic distance due to the chemical and/or the thermal expansion but also by the variation in the oxygen nonsotichiometry and the average valence of cerium ions. (C) 2011 Elsevier B.V. All rights reserved.