Hydrogen peroxide is produced upon radiolysis of water and has been shown to be the main oxidant driving oxidative dissolution of UO2-based nuclear fuel under geological repository conditions. While the overall mechanism and speciation are well known for granitic groundwaters, considerably less is known for saline waters of relevance in rock salt or during emergency cooling of reactors using seawater. In this work, the ternary uranyl-peroxo-chloro and uranyl-peroxo-bromo complexes were identified using IR, Raman, and nuclear magnetic resonance (NMR) spectroscopy. Based on Raman spectra, the estimated stability constants for the identified uranyl-peroxo-chloro ((UO2)(O2)(Cl)(H2O)2-) and uranyl-peroxo-bromo ((UO2)(O2)(Br)(H2O)2-) complexes are 0.17 and 0.04, respectively, at ionic strength ≈5 mol/L. It was found that the uranyl-peroxo-chloro complex is more stable than the uranyl-peroxo-bromo complex, which transforms into studtite at high uranyl and H2O2 concentrations. Studtite is also found to be dissolved at a high ionic strength, implying that this may not be a stable solid phase under very saline conditions. The uranyl-peroxo-bromo complex was shown to facilitate H2O2 decomposition via a mechanism involving reactive intermediates.