It was found that the sintered CeO2, which was not perovskite-type but fluorite-type, permeated hydrogen under H-2-H2O atmosphere at 1050-1800 K. Three tube-shaped specimens (phi 0.287 x 0.894 cm) (SP I, SP II, SP), the lengths of which were 1.40, 0.37, and 1.45 cm, were prepared. The nonexistence of micropores in specimens was confirmed by supplement of He (11.2%) + N-2 to the outside of SP at 1650-1820 K. H-2 (7.90%) + H2O (2.55%) + N-2 was supplied to the outsides of SP I and SP II and Ar gas was allowed to flow to the inside of them. The flow rate to SP I was 21.2 cm(3) min(-1), and the flow to SP II was controlled so that the hydrogen concentrations measured by a gas chromatograph were of the same value. After the calculation of H-2, H2O, and O-2 Concentrations in an equilibrium state inside and outside SP I and SP II and the elimination of mechanically leaked H-2 and permeated hydrogen through the supporting materials (Pt rings, alumina tube, and disk), the atomic hydrogen permeability, J(H), was determined. The hydrogen partial pressures in the outside and inside were 7.90 x 10(-2) and 7.43 x 10(-5) to 1.53 x 10(-3) atm at 1050-1800 K. Log(J(H) mol h(-1) cm(-1)) increases with rising temperature, T, while it is not proportional to 1/T. As the electronic conduction in CeO2 is considerably higher at low oxygen partial pressure, there is a possibility that CeO2 is an electron-proton mixed conductor under H-2-H2O atmosphere at high temperatures.