Barium cerate-based electrolytes have been extensively studied for intermediate-temperature (600 to 800 degrees C) solid-state ionic devices because of their high ionic conductivities at these temperatures. However, the long-term stability of these materials under the conditions of intended applications, such as in a water vapor-containing atmosphere, has been controversial in recent literature. In this study, a gadolinium-doped barium cerate, BaCe0.8Gd0.2O3, has been tested in an atmosphere containing 50 volume percent watervapor at 600 and 700 degrees C for 1000 h. X-ray diffraction, transmission electron microscopy, differential thermal analysis, and electrochemical measurements indicate that there is no creditable evidence to suggest an observable decomposition of BaCe0.8Gd0.2O3 to Ba(OH)(2) and CeO2 under the testing conditions for up to 1000 h, further confirming the stability of the material under fuel cell conditions demonstrated by others.