U(VI) peroxides are important within the nuclear fuel cycle, but reactive oxygen species (ROS) can form upon oxidation. Herein, we identified the spectral signatures of a U(VI) diperoxosuperoxide complex (KUPS-1) and observed that the transformation of U(VI) triperoxide (KUT-1) to superoxide forms occurred with trace-level Cr. U(VI) superoxide complexes were identified in EPR solution spectra without the use of spin-traps.