Abstract: La₂Mo₂O₉ -based oxide-ion conductors have attracted more and more attention since they were reported having high ionic conductivity in 2000 by Lacorre et. al. In this paper the recent investigation on the novel La₂Mo₂O₉-based oxide-ion conductors is reviewed. It is focused on the structure and phase transition, oxide-ion conductivity, mechanism of oxygen vacancy diffusion, chemical stability, and thermal expansion coefficient in the pure and doped La₂Mo₂O₉ samples. It is pointed out that the La₂Mo₂O₉-based oxide-ion conductors could be possibly applied in the field such as solid oxide fuel cells, oxygen sensors, oxygen permeable membrane. ( From : No.1, 2006 Journal of Inorganic Materials )