The crystal structure of fluorite-related Bi3ReO8 containing isolated Real tetrahedra has been studied by powder X-ray and neutron diffraction at elevated temperatures. The phase is cubic and undergoes a phase transition between T similar to 150 degrees C and 250 degrees C passing from space group P2(1)3 at 25 degrees C to I2(1)3 at higher temperature. The material is an oxide ion conductor and presents a sharp increase in conductivity at T similar to 450 degrees C. with sigma similar to 8x10(-3) S cm(-1) above 500 degrees C. Two different long range oxide ion diffusion mechanisms involving vacancy and interstitial defects are proposed. Rotation of isolated ReO4 tetrahedra seem to facilitate the oxide ion conduction. (C) 2012 Elsevier B.V. All rights reserved.