The dissolution behavior of uranyl peroxide studtite, [(UO2)(O-2)(H2O)(2)](H2O)(2), was examined under a wide range of alkaline aqueous environments with and without the addition of hydrogen peroxide. In the absence of added H2O2, studtite dissolved in aqueous solutions with a tetraethylammonium hydroxide to uranium molar ratio greater than 0.5, and the resulting species in solution characterized by Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) is the uranyl peroxide nanocluster U-24, [(UO2)(O-2)(OH)](24)(24-). This is the first demonstration of the formation of uranyl peroxide nanoclusters from studtite in a solution lacking additional hydrogen peroxide. In similar systems containing added hydrogen peroxide (0.01 M 1.0 M), studtite dissolved in solutions with a TEAOH to uranium ratio greater than 0.1, and the resulting uranyl peroxide species in solution was U-28, [(UO2)(O-2)(1.5)](28)(28-).