Conductivity measurements at intermediate temperatures and phase studies were carried out in the compositions Ba x Ce 1−y Gd y O 3−δ (0.8≦x≦ 1.1, 0 ≦ y ≦ 0.3) and Ba x Ce 0.8 Ln 0.2 O 3−δ (x=0.975, 1.0; Ln=Nd,Sm,Gd,Dy,Yb). In Ba x Ce 0.8 Ln 0.2 O 3−δ , the Ba-deficient compositions (x=0.975) showed higher conductivity than the stoichiometric ones (x=1.0) for all the Ln. The total (bulk) conductivities had a maximum (σ=5.5×10 −4 Scm −1 at 250 °C) for Ln=Gd and x=0.975. The σ value was nearly two times higher than that of the known stoichiometric BaCe 0.8 Gd 0.2 O 3−δ electrolyte and the best of those of known oxide ion conductors at intermediate temperatures. The Ba-deficient Ba 0.975 Ce 0.8 Gd 0.2 O 3−δ composition was a single phase of the monoclinic perovskite-related phase (I2/m). The high ionic conductivity is attributed to the fact that the Gd-doped composition had the largest cell volume in the Ln-doped Ba 0.975 Ce 0.8 Ln 0.2 O 3−δ series.