The crystal structures of the two oxides Bi 46 M 8 O 89 (M ¼ P; V) have been solved from single crystals X-ray data at room temperature. Bi 46 P 8 O 89 crystallizes in the monoclinic symmetry (space group C2=m) with the cell parameters a ¼ 19:6073ð4Þ ( A; b ¼ 11:4181ð4Þ ( A; c ¼ 21:1119ð4Þ ( A and b ¼ 112:14ð3Þ1: The symmetry of Bi 46 V 8 O 89 is also monoclinic but the space group is P2 1 =c with the unit-cell parameters: a ¼ 20:0100ð4Þ ( A; b ¼ 11:6445ð4Þ ( A; c ¼ 20:4136ð4Þ ( A and b ¼ 107:27ð3Þ1: Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations (M ¼ P; V) are ordered in the framework. The structures are characterised by isolated MO 4 tetrahedra (M ¼ P; V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms (M ¼ P; V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.