We analyzed the nuclear density distribution in the fast oxide-ion conductor (La0.8Sr0.2)(Ga0.8Mg0.15CO0.05)O-2.8 to explore the conduction path and disorder of oxide ions at high temperatures. The conduction path of oxide ions was not along the straight line between the ideal positions, but exhibited an arc shape away from the site of Ga0.8Mg0.15CO0.05 cations. In the low-temperature rhombohedral phase, the oxide ions were localized near the equilibrium positions, while in the high-temperature cubic structure, they spread over a wide area between the ideal positions. (C) 2003 Elsevier B.V. All rights reserved.