The UV-induced self-assembly of [Mo7O24](6-) to 28-etectron-reduced {Mo-142} at pH 3.4 was monitored by Mo Kedge XAFS spectra of photolytes. The EXAFS curve fit analysis indicates that the accumulation of [MoO4](2-) in an equilibrium with {Mo-8} was accompanied by the formation of two-electron-reduced {Mo-14}, and the subsequent construction of the {Mo-20} and {Mo-21} building blocks provided {Mo-142} through their reductive condensation.