High-temperature phase relation of La0.6Sr0.4Co0.8Fe0.2O3-delta (LSCF) was studied with XRD in controlled atmospheres. The electrical conductivity was measured by the four-probe DC method, at temperatures 973-1173 K, in the Po-2, range of 1-10(-5) Pa. These properties are discussed in combination with oxygen nonstoichiometry and the diffusion constant of oxide ions. It was found that LSCF has a phase transition from rhombohedral to cubic, occurring at about 773 K. The lattice constant of the high-temperature phase was found to increase with decreasing, Po-2, showing the reduction of metallic ions with increased oxygen nonstoichiometry. Although the reduction causes the hole concentration to decrease, hole conduction dominants in the measured Po-3 range. The mobility of holes is almost a constant, 1.17 cm(2)V(-1) s(-1), at 973-1173 K. At 1073 K, the ionic conductivity has a maximum value of 0.08 S cm(-1) under 10(3) Pa of, Po-2 the value is close to that of Gd-doped ceria at the same temperature. (C) 2003 Elsevier Science B.V. All rights reserved.