Brannerite, UTi2O6, can be formed only under low oxygen pressures by dry ceramic processing techniques, but the substitution of similar to0.2 and 0.3 formula units (fu) of Ca or Gd, respectively; for U allows the stabilization of the phase in air. The Ca/Gd in brannerite provides charge compensation for some U to exist in valence states >+4, as found by X-ray absorption spectroscopy of the U L-III-edge, The maximum solubilities of Ca and trivalent rare earths in the air-fired samples, 0.3 and 0.5 fu, respectively, correspond to U having an average valence of +5. Ca and Gd had maximum solubilities of 0.2 and 0.45 fu, respectively, in argon-fired samples. An absorption band at 1448 nm in both air- and Ar-fired U-brannerite doped with Ca and Gd was observed using diffuse-reflectance spectroscopy and attributed to an electronic transition of U5+. A similar band was observed in an annealed natural brannerite, which contained Ca, rare earths, and Th, although the hand was present at similar to 1520 nm in the unannealed, X-ray amorphous sample, In synthetic ThTi2O6 (thorutite, having the brannerite structure), the solubility of Ca was undetectable and that of rare earths <0.1 fu, Other ionic substitutions in synthetic brannerites involved Hf, Pu, La, and Y for U, (Gd + Nb) for U + Ti, and Fe in the Ti site.