High temperature X-ray diffraction measurements were performed under dry and wet atmospheres to investigate phase behavior of BaCe0.8Y 0.2O3-δ (BCY2O). In the temperature range of 30-400 °C, BCY20 was identified to be rhombohedral and monoclinic structures in dry and wet atmospheres, respectively. Larger lattice volumes were obtained in a wet atmosphere due to a chemical expansion induced by water incorporation. A gradual change in diffraction peak shape due to a phase transformation from rhombohedral to monoclinic was observed at 300 °C when moisture was introduced into the atmosphere. These results indicated clearly the dependence of phase behavior of BCY2O on partial pressure of water vapor in atmosphere. © 2013 Elsevier Inc. All rights reserved.