A proton conductor Ba_1.03Ce_0.5Zr_0.4La_0.1O_3－α with a nonstoichiometric composition was prepared by a high temperature solid state reaction. X-ray powder diffraction pattern (XRD) shows that the material was of a single orthorhombic phase of perovskite-type BaCeO₃ and good stability in CO₂ or water steam atmosphere at a high temperature. The SEM morphologies of the surface and the fracture of Ba_1.03Ce_0.5Zr_0.4La_0.1O_3－α show that the material was compact after sintered at 1550 ℃ for 20 h in air. The conductivities and ionic transport numbers of the material in different gases were measured by using ac impendence spectroscopy and gas concentration cell methods in the temperature range of 500～900 ℃, respectively. The results indicate that the material was a pure proton conductor with the protonic transport number of 1 from 500 to 900 ℃ in wet hydrogen. In wet air, the oxide-ionic transport numbers were 0.688～0.170, the protonic transport numbers were 0.218～0.017, and the material was a mixed conductor of oxide-ions, protons and electronic holes. Under the fuel cell condition, the material was a mixed conductor of oxide ions, protons and electrons with the ionic transport numbers of 0.990～0.796 from 500 to 900 ℃. The conductivity and ionic transport numbers of Ba_1.03Ce_0.5Zr_0.4La_0.1O_3－α were higher than those of BaCe_0.5Zr_0.4La_0.1O_3－α under the same experimental conditions.

Key words: Ba_1.03Ce_0.5Zr_0.4La_0.1O_3－α, nonstoichiometry, chemical stability, ionic conduction