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In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)3[PMo12O40] was reacted with nequiv of lithium metal (n= 1–24) to generate PMo12/nproducts which were shown to be mixtures of electron-rich PMo12Lixspecies. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo–Mo bond formation at n∼ 8 and a significant structural change at n> 12. Successive MoVIreductions were monitored by XANES and XPS, and at n= 24, results were consistent with the formation of at least one MoIV–MoIVbonded {MoIV3} triad together with MoV. Upon dissolution, the PMo12Lixspecies present in the solid PMo12/nproducts undergo electron exchange and single-peak 31P NMR spectra were observed for n= 1–12. For n≥ 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV–vis spectra associated with MoV–MoVand {MoIV3} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}+and 2-electron-reduced PMo12in (TBA)4[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li–O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.