The UV-induced self-assembly of [Mo7O24]6- to 28-electron reduced {Mo142} at pH 3.4 was monitored by Mo K-edge XAFS spectra of photolytes. The EXAFS curve fit analysis indicates that the accumulation of [MoO4]2- in an equilibrium with {Mo8} was accompanied by the formation of two-electron reduced {Mo14}, and the subsequent construction of the {Mo20} and {Mo21} building blocks provided {Mo142} through their reductive condensation.