AbstractOxygen storage and release with oxygen diffusion in the bulk of the cerium–zirconium solid solution oxide Ce₂Zr₂O_x (x = 7–8), which possesses an atomically ordered arrangement of cerium and zirconium atoms, is the key to three-way exhaust catalysis. Oxygen storage proceeds via heterogeneous oxygen diffusion into the vacant sites of Ce₂Zr₂O₇ particles, but the heterogeneous oxygen diffusion track is erased after oxygen storage in the Ce₂Zr₂O_x bulk. Here we show three-dimensional hard X-ray spectro-ptychography to clearly visualize the three-dimensional cerium valence map in Ce₂Zr₂O_x particles, and unsupervised learning reveals the concealed oxygen-diffusion-driven three-dimensional nanoscale cerium oxidation behavior and tracking areas inside individual mixed-oxide particles during the oxygen storage process. The described approach may permit the nanoscale chemical imaging of reaction tracking areas in solid materials.