A mixed-valent uranium(IV,VI) diphosphonate, (H3O)(2)(UO2)(3)U(H2O)(2)[CH2(PO3)(2)](3)center dot 6H(2)O (UC1P2S), has been synthesized under hydrothermal conditions. S-2-butanol was used to reduce uranium VI to IV. The tetravalent uranium centers adopt eight-coordinate geometries, while hexavalent uranyl units are all tetragonal bipyramids. The UV-vis-NIR spectrum of UC1P2S shows absorption features for both U(VI) and U(IV).