C-13 and O-17 NMR and Raman spectroscopies were used to monitor the fractions of UO2(CO3)(3)(4-) (1) and (UO2)(3)(CO3)(6)(6-) (2) in aqueous carbonate solutions as a function of pH, ionic strength, carbonate concentration, uranium concentration, and temperature. The multinuclear NMR and Raman data are consistent with the formation of (UO2)(3)(CO3)(6)(6-). The pH dependence of the C-13 NMR spectra was used to determine the equilibrium constant for the reaction 3UO(2)(CO3)(3)(4-) + 3H(+) reversible arrow (UO2)(3)(CO3)(6)(6-) + 3HCO(3)(-), log K = 18.1(+/- 0.5) at I-m = 2.5 m and 25 degrees C, and corresponds to log beta(36) = 55.6(+/- 0.5) for the reaction 3UO(2)(2+) + 6CO(3)(2-) reversible arrow (UO2)(3)(CO3)(6)(6-) under the same conditions. Raman spectra showed the uranyl nu(1) stretching band at 831.6 cm(-1) for monomeric 1 and at 812.5 cm(-1) for trimeric (UO2)(3)(CO3)(6)(6-) (2). EXAFS data from solid [C(NH2)(3)](6)[(UO2)(3)(CO3)(6)] and a solution of (UO2)(3)(CO3)(6)(6-) suggest that the same uranium species is present in both the solid and solution states. Fourier transforms of the EXAFS spectra of both solid and solution samples revealed five well-resolved peaks corresponding to nearly identical near-neighbor distances for solid and solution- samples of 2. Fitting of these peaks yields U-O(uranyl) = 1.79, U-O(carbonate) = 2.45, U- -C = 2.90, U- -O(terminal carbonate) = 4.16, and U- -U = 4.91 Angstrom for the solid Angstrom in both Fourier transforms (uncorrected for phase shift) corresponds to a U- -U interaction at 4.91 Angstrom, a conclusion which is supported by the absence of this peak in the Fourier transform of the crystalline monomeric K-4[UO2(CO3)(3)] Multiple scattering along the uranyl vector is believed to play a significant role in the EXAFS of all three systems.