Zr/Hf aqueous-acid clusters are relevant to inorganic nanolithography, metal-organic frameworks (MOFs), catalysis, and nuclear fuel reprocessing, but only two topologies have been identified. The (Zr-4) polyoxocation is the ubiquitous square aqueous Zr/Hf-oxysalt of all halides (except fluoride), and prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate solution leads to a striking modification of Zr-4, yielding two structures identified by single-crystal X-ray diffraction. Zr-25, isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulated [Zr25O10(OH)(50)(O-2)(5)(H2O)(40)](ClO4)(10)center dot xH(2)O. Zr-25 is a pentagonal assembly of 25 Zr-oxy/peroxo/hydroxyl polyhedra and is the largest Zr/Hf cluster topology identified to date. Yet it is completely soluble in common organic solvents. ZrT d , an oxo-centered tetrahedron fully formulated [Zr-4(OH)(4) (mu-O-2)(2)(H2O)(12)] (ClO4)(6)center dot xH(2)O, is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrTd and Zr-25 in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr-4 undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr-25 that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr-25 from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr-25 and upon redissolution of Zr-25. On the other hand, the 10:1 peroxide/Zr ratio of the ZrTd reaction solution yields larger prenucleation clusters before undergoing peroxide-promote disassembly into smaller fragments. Neither these larger cluster intermediates of ZrTd nor the smaller intermediates of Zr-25 have yet been isolated and structurally characterized, and they represent an opportunity to expand this new class of group IV polycations, obtained by peroxide reactivity and ligation.