Bright, water-soluble CeF3 nanoparticles with small size and narrow size distribution have been synthesized using a simple co-precipitation method without any ligands. Size control of nanoparticles from 13 ± 2 to 9 ± 2 nm was achieved by varying the reaction time. Colloidal properties have been found to vary with pH and, independently, with dilution. The photoluminescence of the as-synthesized nanoparticles shows a highly photostable UV/Visible fluorescence band due to allowed 5d–4f transitions, also observed in the X-ray luminescence spectrum. This band is suitable for X-ray excitation of a range of photosensitizers. The photoluminescence quantum yield of nanoparticles was also determined to be 31 %. Using the measured fluorescence decay time of 25 ns, the radiative lifetime of Ce in CeF3 was found to be 80.6 ns. Both photoluminescence and cathodoluminescence emission are affected by the reaction time and measurement temperature. Electron-beam-induced defect annealing is also observed.