In order to elucidate the uranium solution chemistry at the high HNO3 concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U-IV and U-VI are studied in aqueous HNO3 solutions, as well as in HClO4 solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO4, U-IV exists as a spherical cation of U4+, which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, (UO2)-O-IV center dot nH(2)O, at a lower acidic concentration of 0.1 M HClO4, U-VI exists as a transdioxo uranyl cation, UO22+, and forms a 5-fold pure hydrate complex Of [(UO2)-O-VI(H2O)(5)](2+) in 1.0 M HClO4, With increasing HNO3 concentration, the water molecules of the U-IV and U-VI hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO3-), forming dominant species of [U-IV(H2O)(x)(NO3)(5)](-) in 9.0 M HNO3 and [(UO2)-O-VI(NO3)(3)](-) in 14.5 M HNO3, respectively. The present multitechnique approach also suggests the formation of two intermediate U-VI species, a 5-fold mononitrato complex ([(UO2)-O-VI(H2O)(3)(eta(2)-NO3)](+)) and a 6-fold dinitrato complex ([(UO2)-O-VI(H2O)(2)(eta(2)-NO3)(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO3 concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U-VI complexes is less probable in the present HNO3 system.