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 (Zr4) 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 Zr4, yielding two structures identified by single-crystal X-ray diffraction. Zr25, isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulated [Zr25O10(OH)50(O2)5(H2O)40](ClO4)10·xH2O. Zr25 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. ZrTd, an oxo-centered tetrahedron fully formulated [Zr4(OH)4(μ-O2)2(μ4-O)(H2O)12](ClO4)6·xH2O, is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrTd and Zr25 in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr4 undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr25 that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr25 from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr25 and upon redissolution of Zr25. 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 Zr25 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.