Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF)-based perovskite oxides have attracted much attention as cathode materials with novel catalytic properties in intermediate-temperature solid-oxide fuel cells (IT-SOFCs). The phase stability of these materials, however, is one of the huge obstacles for the commercialization of IT-SOFCs. Here, we examine the long-term stability and the electrochemical properties of La-doped BSCF (BLSCF) in which Ba was partially substituted with La to enhance the phase stability without losing the catalytic activity. From symmetric cell measurements, the initial electrode impedance of BLSCF was found to be 0.04 Omega cm(2) in air at 973 K; it remained nearly constant even after 800 h, in contrast to BSCF without La doping. It was further demonstrated that a large tubular cell consisting of a BLSCF cathode exhibited a high maximum power density of 0.52 W cm(-2) and impressive long-term stability at 973 K. Free-energy calculations using density functional theory and XRD experiments for these cathodes showed that the addition of La into BSCF cathode makes the cubic structure of the cathode stable and the phase transition into the hexagonal phase is prohibited. The excellent electrochemical activity of BSCF based cathode is maintained over 950 h in a large tubular cell. (C) 2013 Elsevier B.V. All rights reserved.