Cerianite-(Ce), ideally CeO2, occurs as rounded grains up to 5 μm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 μm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) Å) and Raman spectroscopy (minor vibration modes at 184 and 571 cm−1 in addition to a strong signal at 449 cm−1) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.9–15.5 wt.% LREE2O3 and 4.9–9.7 wt.% HREE2O3), and that it is enriched in yttrium (7.1–14.5 wt.% Y2O3) and fluorine (2.2–3.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cation–anion distance of 2.3471(6) Å. The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks.