The tetramethylammonium counterion was used to suppress formation of insoluble uranate salts, M2U2O7, and allow for a detailed structural and spectroscopic characterization of UO2(OH) n 2- n (n = 4, 5) under highly alkaline aqueous solution conditions. Single crystals of Co(NH3)62UO2(OH)43·H2O were obtained by cooling a dilute solution of Co(NH3)6Cl3 and UO2(NO3)2·6H2O in 3.5 M (Me4N)OH to 5 °C. The asymmetric unit contains three distinct UO2(OH)4 2- ions, each displaying a pseudo-octahedral coordination geometry with trans oxo ligands. The three independent UO2(OH)4 2- ions in the unit cell give average UO and U−OH distances of 1.82(1) and 2.26(2) Å, respectively. EXAFS data on solid Co(NH3)62UO2(OH)43·H2O and aqueous UO2 2+ in 3.5 M (Me4N)OH solution were collected at the U LIII edge, and the resulting radial distribution function shows a single asymmetric peak. For the solid and solution, curve fitting reveals two near neighbors. For the crystalline solid, the first shell was fit with two O atoms at a distance of 1.81(1) Å, and the second shell was fit with 3.9(5) O atoms at a distance of 2.21(1) Å. For the solution sample, the first shell contains two O atoms with a UO distance of 1.79(1) Å, and the second O shell was fit with 5.2(5) O atoms at a U−O distance of 2.22(1) Å. The bond distances for both the solution and solid state samples correspond relatively well with the single-crystal diffraction data; however, the second-shell coordination number is larger in solution than in the solid state, indicating a greater number of OH ligands in solution than in the solid state. Both EXAFS and X-ray diffraction analyses reveal relatively long axial UO and short equatorial U−OH bonds. Raman spectra of single crystals of Co(NH3)62UO2(OH)43·H2O reveal a symmetrical OUO stretch at 796 cm-1, 74 cm-1 lower than that for the uranyl aquo ion. In solution, the symmetrical OUO stretch is at 786 cm-1, 10 cm-1 lower than observed in the solid state. 18O enrichment produces a shift to 752 cm-1 confirming the assignment in solution. Luminescence spectroscopy recorded as a function of hydroxide ion concentration reveals that an equilibrium exists between two species, assigned to UO2(OH)4 2- and UO2(OH)5 3-. The vibronic structure of the luminescence bands was used to determine a vibrational energy of 790 cm-1 for UO2(OH)5 3- to confirm its assignment. 17O NMR and 16O/18O Raman spectroscopies also reveal an unprecedented facile ligand exchange between UO and bulk solvent oxygen atoms. Line-broadening analysis of the 17O NMR data provide activation parameters of ΔH ⧧ = 9.8 ± 0.4 kcal/mol, ΔS ⧧ = −18 ± 6 cal/mol·K, and k ex 298K = 45 ± 15 s-1. Crystal data for Co(NH3)62UO2(OH)43·H2O:  monoclinic space group C2/c, a = 17.4130(4) Å, b = 12.1794(3) Å, c = 15.3721(4) Å, β = 120.384(1)°, Z = 4, R1 = 0.0313, wR2 = 0.0734.