A neutron diffraction study was performed on (Bi2O3)O.80(Er2O3)0.20 in the temperature region of 300–1100 K. There is no long-range ordering of vacancies. It is concluded from diffuse scattering that at low temperatures short-range ordering appears, leading to the occurrence of relatively short Ln-O distances. At temperatures above 870 K the oxygen lattice disorders. In the low temperature region of Bi2O3-Ln2O3 solid solutions with the δ-Bi2O3 structure the activation energy of the conductivity is determined by the strength of the Ln-O band. In the high temperature region the energy necessary for oxygen ions to migrate through the tetrahedron planes plays a role.