Niobium oxides are used as photocatalysts and also used to sequester radionuclides from waste. Hydrogen peroxide is a common byproduct from these applications, and herein we examine how isopolyoxoniobates, the hexaniobate [HxNb6O19](8 x) and the decaniobate [HxNb10O28](6 x) ions, react with hxdrogen peroxide. Both the isopolyoxoniobate and the peroxopolyoxoniobate species can be detected by using electrospray-ionization mass spectrometry (ESI-MS) and 17O NMR spectroscopy. Interestingly, we observed that although the decaniobate anion rapidly picks up peroxide, the peroxodecaniobate clusters can dissociate over time and yield the peroxohexaniobate species, and sometimes in the presence of reformed and peroxy-free decaniobate ion. In both isopolyoxoniobate structures, the reaction proceeds by the replacement of niobium terminal oxygen atoms with h2- O(I)22 . The peroxohexaniobate species are extraordinarily stable and we have determined the crystal structure of [N(CH3)4]5[H3Nb6O13(O(I)2)6]A?9.5H2O, in which all six terminal oxygen atoms are replaced by h2-O(I)22 . This crystal structure us the first example of an isolated peroxoisopolyoxoniobate cluster.