Although Al(III) hydrolysis, condensation, and nucleation play pivotal roles in the synthesis of Al-based compounds and determine their chemical behavior, we still lack experimental evidence regarding the chemistry of nucleation from solution. Here, by combining advanced titration assays, high-resolution transmission electron microscopy (HR-TEM), and 27Al-nuclear magnetic resonance spectroscopy, we show that highly dynamic solute prenucleation clusters (PNCs) are fundamental precursors of nanosolid formation. Chemical changes from olation to oxolation bridging within PNCs rely on the formation of tetrahedral AlO4 in solution and trigger phase separation at low driving force (supersaturation). This does not include the formation of Keggin-Al13 ions, at least during the earliest stages. The PNC pathway of the formation of Al(III) (oxy)(hydr)oxides offers new possibilities toward the development of strategies for controlling the entire crystallization process. © 2020 The Authors.