Li- and Ag-doped Bi2VO5.5 were synthesized by solid-state reactions, and their high oxide-ion conductivities at intermediate temperatures between 400 and 600 degrees C were demonstrated. Li- and Ag-doped Bi2VO5.5 show a higher symmetry than Bi2VO5.5 at room temperature. In Bi-2(V0.9Li0.1)O-5.3 and Bi-2(V0.9Ag0.1)O-5.3, the tetragonal gamma-phase, i.e., the high-temperature and disordered phase of Bi2VO5.5, is stabilized at room temperature. AC impedance spectroscopy reveals that Li- and Ag-doping markedly improve the electrical conductivity of Bi2VO5.5 below 570 degrees C. The dominant charge carrier is confirmed as oxide ions from impedance spectra, DC polarization measurements, and conductivity change with oxygen partial pressure. At around 500 degrees C, the oxide-ion conductivities of Li-and Ag-doped Bi2VO5.5 are 2-5 times higher than that of La0.8Sr0.2Ga0.8Mg0.115Co0.085O3, which is one of the best oxide-ion conductors. (C) 2010 Elsevier B.V. All rights reserved.