A comparative analysis is made of the photoredox, electrochemical and electron storage properties of polyoxotungstate and polyoxomolybdate clusters in comparison with those of colloidal WO3 particles. Both species can harvest light and act as photocatalysts, but polyoxotungstates present a smaller size, wider gap, lower reduction potentials and greater tunability. Furthermore, WO3 colloids are structurally ill-characterized and tend to coagulate whereas polyoxometalate structures are well characterized, controllable and stable in solution. These characteristics and the confinement of injected electrons in a zero-dimensional nanometre-sized polynuclear molecule make of polyoxometalates remarkable models for colloidal quantum-sized semiconducting oxides.