Superstructuring in the new compound Fe_1.89Mo_4.11O₇ has been elucidated by transmission electron microscopy. This compound is a member of the family M₂MO₄O₇ and has both iron and molybdenum atoms occupying octahedrally coordinated sites in the structure, represented by Fe^t(Fe_0.89M_0.11)⁰Mo₄O₇. The superstructuring, detected only by electron diffraction, involved tripling of all three lattice parameters of the subcell. The subcell was structured by single crystal x-ray diffraction [Imma, no. 74, a = 5.9793(5) Å, b = 5.7704(4) Å, and c = 17.036(1) Å]. This structure type contains a close-packed arrangement of Mo₄O₇ units, which are infinite chains of trans edge-shared molybdenum octahedra running parallel to b*. Two different coordination environments are observed for the cations. Parallel to the a* direction, infinite edge-sharing MO₆ (M = 89% Fe or 11% Mo) octahedra are observed. The second cation site, with nearly tetrahedral coordination by oxygen, is filled solely by iron. The superstructure can be rationalized by a regular arrangement of iron and molybdenum atoms in the octahedrally coordinated cation sites.