The stability of Gd-doped and La-doped BaCeO3 in water-containing atmospheres was examined. Samples containing up to 20 mol % Gd and 30 mol % La were synthesized by calcining the requisite mixtures of precursors. Three types of tests were performed: (i) thermal treatment in an atmosphere containing water vapor (p(H2O) = 0.0964 atm) between 150 and 400 degrees C, (ii) boiling in water, and (iii) simultaneous exposure to boiling water and water vapor at essentially the same temperature. Samples were characterized by X-ray diffraction. All powders boiled in water decomposed within a few hours, whereas none of the powders decomposed when exposed to similar to 1 atm water vapor pressure at the same temperature. This difference was rationalized on the premise that the mechanisms of decomposition depend upon the form of water, liquid or vapor. In liquid water, the decomposition kinetics appear to be interface-controlled. By contrast, in water vapor, the kinetics appear to be dictated by bulk diffusion of water into the lattice.