Rare earth oxide doped BaCeO3 is known to exhibit high protonic conductivity in the temperature range 500 to 900 degrees C and is a potential electrolyte for use in hydrogen sensing and fuel cell applications. Prior work, however, has shown that BaCeO3 may be thermodynamically unstable at low temperatures. In the present work, the stability of BaCeO3 in an H2O vapor containing environment was investigated by exposing powder and sintered samples to similar to 430 Torr H2O in the temperature range 500 to 900 degrees C. All BaCeO3 samples decomposed into CeO2 and Ba(OH)(2) in relatively short periods of time at temperatures less than 900 degrees C. Doped BaCeO3 decomposed at a faster rate than the undoped BaCeO3. Sintered BaCeO3 decomposed at a rate comparable to the powder samples. These results establish that BaCeO3 is thermodynamically unstable when a critical H2O vapor pressure is exceeded and that the rapid decomposition of both powder and sintered samples is the result of the high solubility of H2O in BaCeO3 which accelerates the kinetics of decomposition.