The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)(24)(O-2)(24)(P2O7)(12)][IO3](2)center dot 140H(2)O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D(4)h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. P-31 NMR measurements demonstrate retention of this symmetry in solution, while in situ P-31 NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.