The atomic hydrogen (H) permeability, J(H), of 10YSZ single crystal (fluorite-type) was measured at 1800-1050 K using the Nigara-method. Two tubular specimens (phi 1.30x0.93 cm; length: 3.35 cm (Specimen(L)) and 1.00 cm (Specimen(S))), annealed at 1570 K for 6 h in air, were used to eliminate the permeations of supporting materials (Pt rings, alumina tube, and alumina disk). Measurement was carried out after the specimen had been kept at 1800 K for 20 h. The J(H) was 3.97x10(-7) -2.79x10(-9)/mol h(-1) cm(-1), when it was assumed that P-H2=7.36x10(3) Pa and P-H2=2.30x10(3) Pa outside of both specimens and that the Ar flow rates to the inside of Specimen(L) and Specimen(S) were 21.5 and 16.9-19.9 cm(3) min(-1), respectively. Log J(H) decreased with the decrease of temperature and was proportional to the inverse temperature. The activation energies were 0.61+/-0.09, 1.60+/-0.05, and 0.79+/-0.02 eV at 1800-1680, 1680-1330, and 1330-1050 K, respectively. The protonic conductivity, which was roughly calculated from the J(H) and the hydrogen partial pressures inside and outside of the specimens, was 2.4x10(-6)-1.5x10(-8)/S cm(-1) at 1800-1050 K. The J(H) of other 10YSZ single crystal specimens, which were not annealed and which were measured after being kept at 1800 K for 1h, was lower than that of the above specimens, especially at high temperatures, and the activation energies were 1.27+/-0.03, 0.15+/-0.03, and 1.49+/-0.09 eV at 1800-1425, 1425-1150, and 1150-1050 K, respectively. The 10YSZ single crystal would be an electron-proton mixed conductor under H-2-H2O atmosphere at high temperatures. Since the J(H) of 12YSZ single crystal specimens, which were annealed at 1870 K for 24 h in air and were measured after being kept at 1800 K for 18 h, was less than I x 10-9/mol h(-1) cm-1 at 1800 K, it was difficult to determine the relation between its hydrogen permeability and temperature. (C) 2003 Elsevier B.V. All rights reserved.