While the majority of studies of alteration of UO2 and commercial spent nuclear fuel under simulated geological repository conditions have emphasized the importance of uranyl oxide hydrates and uranyl silicates, the influence of peroxide on repository performance has been largely overlooked. There is considerable evidence that uranyl peroxides will be important alteration phases of nuclear waste, and that these phases may impact the long-term performance of a geologic repository such as Yucca Mountain. Here we report the thermodynamics and kinetics of becquerelite, Ca[(UO2)6O4(OH)6](H2O)8, in the presence of solutions containing hydrogen peroxide. Thermodynamic calculations reveal that in solutions containing 3.5 × 10-6 M hydrogen peroxide, studtite is thermodynamically favorable over becquerelite at 298 K. To access the kinetics of this reaction, batch experiments were conducted by the reaction of becquerelite and solutions containing hydrogen peroxide. In the presence of 0.1 M hydrogen peroxide, becquerelite altered to studtite within eight hours.