Three novel uranyl(VI) peroxide complexes, [(UO2)(2)(CH3COO)(2)(O-2)(C10H8N2)] (1), [(UO2)(2)(CH3COO)(2)(O-2)(C12H12N2)] (2), and [(UO2)(3)(CH3COO)(4)(O-2) (C15H11N3] (3), have been synthesized and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and luminescence spectroscopy. Each of these structures feature a [(UO2)(2)(O-2)] dimer with additional coordination by acetate, 2,2'-bipyridine (BP), 5,5'-dimethyl-2,2'-bipyridine (MeBPY), or 2,2':6,2 ''-terpyridine (TPY). Compound 3 consists of an additional uranyl unit functionalized with a TPY donor ligand. The presence of the peroxo ligand in 1-3 is due to in situ generation of peroxide when preparative solutions of 1-3 were exposed to ambient light and/or sunlight.