Abstract Dense sedimentary iron formations (IFs) subducted into the mantle during Earth’s early history, may have descended to the core‐mantle boundary where they could exert strong control on its thermal and seismological properties. A key unanswered question is the extent to which IFs retain their oxidized character in the much more reducing mantle. Here we present new experimental constraints on the kinetics of iron oxide reduction under deep subduction conditions (1.5–15 GPa, 883–1,183 K). The reduction of hematite and magnetite to wüstite is controlled by diffusional transport of iron through the wüstite, and is rapid enough to enable complete reduction of ferric iron to ferrous iron in IFs during subduction. Therefore, if IFs are to explain the density and P‐ and S‐wave velocities of ultra‐low velocity zones (ULVZs) at the base of the mantle, they likely must be able to do so without including substantial amounts of ferric iron.