Calcium fluoride single crystals doped with uranyl nitrate were grown by an improved temperature gradient technique under different conditions. Absorption spectra, energy levels and unit cell parameters were studied to analyze the possible color centers and valence states of uranium ions in as-grown U:CaF2 crystals. Uranium in U:CaF2 crystals grown in the presence of PbF2 as an oxygen scavenger is trivalent. F-centers and other defects related to oxygen, with respective absorption lines at 604 and 526 nm, and impure valence states of uranium ions exist in U3+:CaF2 when the molar ratio of PbF2 to U is less than 25. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside to outside where the red part in the core is still U3+:CaF2. Mixed valence states of uranium ions exist in the crystal. The valences of uranium ions are inferred to gradually increase from + 3 to + 6 according to the graded changes of the absorption spectra and unit cell parameters. (C) 2004 Elsevier B.V. All rights reserved.