Two bismuth oxide materials containing large lanthanide and rhenium cations, Bi₂₅La₃Re₂O₄₉ and Bi₂₅Pr₃Re₂O₄₉, have been synthesised in the stabilised face-centred cubic structure. They exhibit very high oxide ion conductivity but prolonged annealing at 600 °C causes a decrease in conductivity, which has been shown to relate to an order–disorder phase transformation. The details of this transition have been determined using neutron powder diffraction on annealed samples. Upon annealing, the samples undergo cation ordering to form a tetragonal phase (I4/m, a ≈ 8.7 Å, c ≈ 17.4 Å), which is structurally related to the unsubstituted phase, Bi₂₈Re₂O₄₉. Neutron powder diffraction refinement reveals that the lanthanide cations enter only one of the three Bi sites in the ordered structure.