Conductivity measurements at intermediate temperatures and phase studies were carried out in the compositions BaxCe1-yGdyO3-delta (0.8 <= 5x <= 5 1.1, 0 <= y <= 0.3) and BaxCe(0.8)Ln(0.2)O(3-delta) (x= 0.975, 1.0; Ln =Nd,Sm,Gd,DyYb). In Ba(x)Ce(0.8)Ln(0.2)O(3-delta), 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 (sigma = 5.5 x 10(-4) Scm(-1) at 250 degrees C) for Ln= Gd and x= 0.975. The sigma value was nearly two times higher than that of the known stoichiometric BaCe0.8Gd0.2O3-delta electrolyte and the best of those of known oxide ion conductors at intermediate temperatures. The Ba-deficient Ba0.975Ce0.8Gd0.2O3-delta composition was a single phase of the monoclinic perovskite-related phase (12/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-delta) series. (C) 2007 Elsevier B.V All rights reserved.