Data on the electrochemical and photoredox chemistry of polyoxometalates of formula H-4[SiW12O40] and H-3[PMo12O40] are reported and analyzed in relation to those of related extended solids (WO3 and colloidal WO3 particles), These soluble multinuclear anions show reversible single-electron and multielectron reduction processes to yield blue mixed valent species. Similar reduced species can be obtained photochemically, with a concomitant oxidation of organic substrates. The reported photochromic behavior is similar to that of colloidal WO3 particles. On the other hand, electrochemical potentials differ substantially from the flat band potential of WO3 colloids. Finally, the title compounds are more stable and present the added advantage of having well-characterized monodisperse and controllable structures and tunable electrochemical properties.