Polymolybdates(VI) have been photochemically reduced to Mo^v in aqueous solutions containing acetic acid. Carbon dioxide, methane, and succinic acid were isolated as decomposition products of the acetic acid but the quantum yield of their formation was ca. 20 times smaller than that of Mo^v formation. E.s.r, spin-trapping experiments exhibited the presence of spin-adducts of hydroxyl radical as an oxidation product of water. The photoredox reaction of the polymolybdate(VI) with water was a main process. The reaction mechanism discussed is based on the similarity with the photochemistry for an alkylammonium polymolybdate(VI). The photoredox reaction of the polymolybdate(VI) with water or acetic acid proceeds via the photoinduced formation of a charge-transfer complex between the polymolybdate(VI) and acetic acid, which is preceded by the transfer of an acetic acid proton to an oxygen atom at a photoreducible octahedral site in the polymolybdate(VI).