Polyoxometalates are photosensitive at the near visible and UV areas, in presence of a great variety of organic compounds. The photosensitivity results in oxidation of organic compounds, with concomitant stepwise reduction of polyoxometalates. Generally the photoredox reactions proceed until another species is able to accept the electrons, thus closing the photocatalytic cycle. Dioxygen plays a key role in photocatalysis by polyoxometalates: (a) it reoxidizes (regenerates) the catalyst very fast and effectively, and/or (b) through its reductive activation by the reduced polyoxometalate (formation of superoxy, O₂^-, and hydrosyperoxy, HO­₂, radicals), causes further selective or nonselective oxidations of a great variety of organic compounds.