In this work, cerium dioxide nanoparticles (CeNPs) were synthesized by thermal decomposition of cerium oxalate decahydrate Ce2(C2O4)3·10H2O. Although a large amount of water was removed along with CO2, the product of decomposition retained the shape of the precursor crystal, i.e. a pseudomorph formed. The Ce2(C2O4)3·10H2O precursor crystals were of plate-like shape; this shape was inherited by CeO2 formed upon decomposition of the precursor. CeO2 plates with a thickness of less than 15 µm were transparent to visible light. X-ray diffraction analysis, transmission electron microscopy and specific surface area measurements indicated that the plates consisted of CeNPs 4–6 nm in size. The influence of the annealing temperature on the growth of crystallites of the pseudomorph was studied. Fast coarsening of the crystallites started at a temperature of 500 °C.