Conventionally, oxide ion conduction in ceramics is viewed as requiring high-symmetry crystal structures, preferably cubic or near cubic, such as the fluorite- [1,2] and perovskite-type oxides.[2]In materials with these structure types it is accepted that ion conduction is isotropic and that this property is essential for the application of these materials in useful devices such as solid oxide fuel cells (SOFCs) and permeation membranes. Indeed the development of fast oxide ion conductors has been predicated on high-symmetry systems. More recently, fast ion conduction has been demonstrated in anisotropic oxides such as La 2 NiO 4þd (LNO) [3][4][5][6][7] and GdBaCo 2 O 5þd (GBCO). [8][9][10][11] In these anisotropic systems, excellent performance as ionic conductors and as fuel cell electrodes [10,12,13] has been demonstrated for the bulk materials, suggesting that the ion conduction in the anisotropic direction is not adversely affected by the relatively slow transport perpendicular to this conduction plane.[3] Each of these compositions has lower symmetry than the typical fluorite-and perovskite-type oxide ion conductors such as La 2 NiO 4þd and related layered materials adopting crystallographically tetragonal and/or orthorhombic structures depending upon composition. With La 2 NiO 4þd in the orthorhombic structure the diffusivity of the oxygen species has been shown, from atomistic simulations, to involve oxygen interstitial species; [14] a result that has been confirmed experimentally from neutron diffraction studies of the Pr analogue. [7] In both the LNO and GBCO families the materials adopt crystallographic structures with relatively low symmetry that cannot be considered as distorted cubic systems. Clearly, these materials show that fast ion transport is feasible in lower symmetry systems and occurs at levels that are competitive with the high-symmetry three-dimensional ionic conductors. Having demonstrated fast oxide ion conduction in noncubic systems (GBCO) and those with oxygen excess (interstitial) contents (LNO), efforts have subsequently been focused on widening the range of new, fast oxide ion conductors and evaluating the potential of structurally complex materials. Here, we report on remarkable low-temperature fast ion conductivity in a complex modulated, superstructured, interstital oxide, CeNbO 4þd .CeNbO 4þd has been reported to exist in one of four polymorphs depending on excess-oxygen stoichiometry, with discrete phases reported for the CeNbO 4 , CeNbO 4.08 , CeNbO 4.25 , and CeNbO 4.33 compositions.[15] Structurally CeNbO 4þd adopts the parent fergusonite monoclinic structure at temperatures below 800 8C and, upon oxidation of Ce 3þ to Ce 4þ , a long-range ordered oxygen sublattice exists, resulting in a series of complex polymorphs including both commensurate and incommensurate superstructures, as evidenced by electron diffraction. [15] Previously, as an extension of studies on anisotropic oxide ion conducting La 2 NiO 4þd -type oxides, the high-temperature scheelite-type CeNbO 4þd phase w...