The crystal structure of the new fast oxide-ion conductor La2Mo2O9 (ionic conductivity of 0.06 S cm-1 at 800 °C) has been studied. This compound presents a reversible phase transformation around 580 °C from a low-temperature form α-La2Mo2O9 to a high-temperature form β-La2Mo2O9. The high-temperature form β-La2Mo2O9 has a cubic structure (at 617 °C, space group P213; a = 7.2014(5) Å; Z = 2; RBragg = 5.8%, Rp = 10.9%, Rwp = 6.5%, χ2 = 7.7) which derives from that of β-SnWO4. Partial site occupation by oxygen atoms, strongly anisotropic thermal factors, and short-range order with a distance characteristic of O−O pairs have been evidenced. An original concept is proposed for the origin of oxide−ion conduction in this compound, which could be applied to the design of new oxide−ion conductors. The low-temperature form α-La2Mo2O9 exhibits a slight monoclinic distortion and a large superstructure relative to β-La2Mo2O9 (2 × 3 × 4), most probably due to the localization of oxygen atoms. The large cell (∼8800 Å3) did not allow us to determine the crystal structure of α-La2Mo2O9.