The atomic hydrogen (1/2H(2)) permeability, J(H), in (CeO2)(0.9)(GdO1.5)(0.1) (fluorite-type) was measured at 1800-800 K. Two tubular specimens (SP(L), SP(S)) of different lengths sintered at 1970 K in air were used to eliminate the permeation through supporting materials (Pt rings, alumina tube and alumina disk). The J(H) was 2.81 X 10(-6)-1.50 x 10(-8)/mol h(-1) cm(-1), when it was assumed that P-H2 = 7.34 x 10(3) Pa. and P-H2O = 2.28 x 10(3) Pa outside of both specimens and that the Ar flow rates to the inside of SP(L) and SP(S) were 20.9 and 9.4-16.0 cm(3) min(-1), respectively. LogJ(H) decreased with decreasing temperature and was proportional to the inverse temperature at 1800-1100 and 1100-800 K, and the activation energies were 1.12 +/- 0.01 and 0.18 +/- 0.01 eV, respectively. (CeO2)(0.9)(GdO1.5)(0.1) would be an electron-proton mixed conductor under H-2-H2O atmosphere at high temperatures. The protonic conductivity was roughly calculated from the JH and from the hydrogen partial pressures inside and outside the specimens, its values being 1.4 x 10(-4)-7.2 x 10(-7)/S cm(-1) and 1.2 x 10(-6)/S cm(-1) at 1070 K.