Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS) has been used to study the speciation of uranium(VI) at very alkaline pH. The hydroxocomplex UO2(OH)3 has been reported to be responsible for all uranium fluorescence at pH 12. In this conditions hydrogen peroxide has a dramatic influence in the fluorescence of UO2(OH)3, known as quenching. Observing fluorescence lifetime we have concluded that only static quenching occurs. The study of the variation of the fluorescence as a function of hydrogen peroxide concentration in solution, has allowed the determination of the formation constant of the uranyl-peroxide complex UO2(OH)2(O2)-, with logK0=29.3 ± 0.4. In addition to TRLFS experiments, formation of uranyl-peroxide complexes has been studied with UV-Vis (UV-Visible light) spectrophotometry. Two complexes have been found in the H2O2/U(VI) ratio < 2. Graphical and numerical methods have been used to determine the equilibrium constants. For the numerical determination program STAR has been chosen, due to his capacity of treating multiwavelength absorbance data and refining equilibrium constants. The logarithm of the equilibrium constant is found to be 27.9 ± 0.1 for the first specie (UO2O2(OH)2 2-) and 37.1 ± 0.2 for the second specie (UO2(O2)2(OH)2 4-). The formation of those complexes in solution has a very important impact on the speciation of uranium(VI), because it is predominant at hydrogen peroxide concentrations higher than the total uranium concentration, at alkaline pH and, for example, increases the solubility of the uranyl peroxide studtite more than two orders of magnitude depending on the solution conditions.