The composition and equilibrium constants of the complexes formed in the binary U(VI)–hydroxide and the ternary U(VI)–hydroxide–peroxide systems have been studied using potentiometric and spectrophotometric data at 25 °C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO₂(OH)(O₂)]⁻ and [(UO₂)₂(OH)(O₂)₂]⁻. Under our experimental conditions the former is predominant over a broad p[H⁺] region from 9.5 to 11.5, while the second is found in significant amounts at p[H⁺] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO₂)₂(OH)(O₂)₂]⁻ was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M⁻¹ cm⁻¹, respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO₂)(OH)(O₂)]⁻ at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO₂²⁺(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO₂)(OH)(O₂)]⁻ might be the building block in cluster compounds such as [UO₂(OH)(O₂)]₆₀⁶⁰⁻ studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173–2177). Speciation calculations using the known equilibrium constants for the U(VI) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by α-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.