It was found that hydrogen permeated Ce0.8Yb0.2O1.9 (fluorite-type) at 1800-1000 K, which was sintered at 1970 K in air. The tube shared two specimens were used to measure the hydrogen permeability, J(H). Log(J(H)/mol h(-1) cm(-1)) decreases with reducing temperature and is proportional to reverse temperature at 1800-1500, 1500-1300 and 1200-1000/K. The activation energies were 0.92+/-0.06, 1.10+/-0.06 and 0.58+/-0.05/eV, respectively. Ce0.8Yb0.2O1.9 would be an electron-proton mixed conductor under H-2-H2O atmosphere at high temperature.