Hydrogen permeation properties of Gd_0.1Ce_0.9O_x/BaCe_0.80Y_0.20O_3−δ (GDC/BCY) dual-phase membranes were evaluated. Nongalvanic hydrogen permeation was observed for 46 vol % GDC-containing BCY membranes: the permeation rate was 0.30 ml·min⁻¹·cm⁻² at 800°C. This is almost comparable to the reported value of Ni–BCY cermet (0.50 ml·min⁻¹·cm⁻²). This nongalvanic hydrogen permeation unambiguously indicates that the BCY–GDC composite functions as a mixed proton–electron conductor in an H₂ reducing atmosphere. From a conductivity analysis, it was deduced that 46 vol % GDC/BCY possesses proton and electron conductivities comparable to the bulk states of BCY and GDC. The composite membrane was stable in an atmosphere switching between H₂ and air. TGA analysis indicated that addition of GDC enhanced the stability of the BCY phase in CO₂.