Complex formations in aqueous solutions (pH = 9.7∼11.8) containing uranyl ions (5 or 20 mM, M = mol dm -3 ) carbonate ions (0.04, 0.12, or 1.05 M), and hydrogen peroxide have been studied using UV-vis and NMR spectroscopic methods. In 17 O NMR spectra, a peak due to uranyl “yl” oxygen of UO 2 (CO 3 ) 3 4− was observed at 1099 ppm, and with an increase in [H 2 O 2 ], the intensity of this peak decreases and two peaks were newly appeared at 1094 and 1103 ppm, respectively. In 13 C NMR spectra, a carbonate peak of UO 2 (CO 3 ) 3 4− and two peaks due to other carbonate complexes were observed at 170.4, 169.8, and 169.9 ppm, respectively. In addition, it was found that the peak intensities of 17 O signals observed at 1094 and 1103 ppm change in proportion to those of 13 C signals observed at 169.9 and 169.8 ppm. From the NMR peak areas, two new uranyl complexes were proposed as species with the molar ratio of UO 2 2+ : CO 2- =1: 2. Furthermore, the UV-vis absorption spectral changes with the addition of H 2 O 2 to the aqueous solutions containing uranyl ions and carbonate ions were analyzed by the least-square fitting on the basis of the formation of UO 2 (O 2 )(CO 3 ) 2 4− and (UO 2 ) 2 (O 2 )(CO 3 ) 4 6- complexes except for UO 2 (CO 3 ) 3 4- . As a result, the formation constants (logK) for UO 2 (O 2 )(CO 3 ) 2 4− and (UO 2 ) 2 (O 2 )(CO 3 ) 4 6− were estimated as 7.9 ± 0.6 and 31.9 ± 0.6, respectively. Using the obtained formation constants, the molar absorptivity of UO 2 (O 2 )(CO 3 ) 2 and (UO 2 ) 2 (O 2 )(CO 3 ) 4 6- were evaluated. It was found that the UO 2 (O 2 )(CO 3 ) 2 4- complex has an absorption peak at 350.6 nm and a shoulder around 420.0 nm, and their ɛ values are 1260 ± 21 and 893 ± 18 M -1 cm -1 , and that the absorption spectrum of (UO 2 ) 2 (O 2 )(CO 3 ) 4 6- have a shoulder around 346.0 nm (ɛ = 1720 ± 24).