The diffusion coefficient and amount of water in trivalent cation doped barium zirconate were investigated by thermogravimetry. The apparent diffusion coefficient of water of yttrium-doped barium zirconate which was sintered for 24 hours, Y24, was 2.9 × 10-11 m2 s-1 at 700 °C under pH2O= 0.05 atm. The diffusion coefficient is almost independent of the sintering period. This means that the diffusion coefficient is independent of the grain size. The ratio of water to oxide ion vacancy before water introduction was evaluated from weight change of water in thermogravimetry. This ratio in Y24 is about 32 % and 62 % at 600 °C and 500 °C, respectively. Electric conductivity of sintered pellets was also measured. In S24, which is scandium-doped barium zirconate sintered for 24 hours, the change of the bulk conductivity after change of the atmosphere from dry to wet was governed by the diffusion of water. It takes more than 10 hours to thermodynamically complete full hydration of the specimen in the disk with the diameter of 10 mm and thickness of 1.5 mm even at 700 °C.