Nanoscale Fe : CeO_2−x oxygen storage material for the process of chemical looping has been investigated by advanced transmission electron microscopy and electron energy-loss spectroscopy before and after a model looping procedure, consisting of redox cycles at heightened temperature. Separately, the activity of the nanomaterial has been tested in a toluene total oxidation reaction. The results show that the material consists of ceria nanoparticles, doped with single Fe atoms and small FeO_x clusters. The iron ion is partially present as Fe³⁺ in a solid solution within the ceria lattice. Furthermore, enrichment of reduced Fe²⁺ species is observed in nanovoids present in the ceria nanoparticles, as well as at the ceria surface. After chemical looping, agglomeration occurs and reduced nanoclusters appear at ceria grain boundaries formed by sintering. These clusters originate from surface Fe²⁺ aggregation, and from bulk Fe³⁺, which “leaks out” in reduced state after cycling to a slightly more agglomerated form. The activity of Fe : CeO₂ during the toluene total oxidation part of the chemical looping cycle is ensured by the dopant Fe in the Fe_1−xCe_xO₂ solid solution, and by surface Fe species. These measurements on a model Fe : CeO_2−x oxygen storage material give a unique insight into the behavior of dopants within a nanosized ceria host, and allow to interpret a plethora of (doped) cerium oxide-based reactions.