Polyoxometalates appear to be effective photocatalysts for a variety of organic pollutants, leading to their decomposition to CO₂ and H₂O and Cl⁻ in case of chlorinated hydrocarbons. Various aromatic derivatives and chlorinated acetic acids, undergo mineralization upon photolysis with near visible and UV light, in presence of W₁₀O⁴⁻₃₂, PW₁₂O³⁻₄₀, and SiW₁₂O⁴⁻₄₀. These catalysts are, at least, as effective as TiO₂. The main oxidant appears to be OH radicals formed by the reaction of the excited polyoxometalates with H₂O. At the initial stages of photoreaction, dioxygen has a profound effect (up to eighteen times faster) on the photodegradation of chloroacetic acid, whereas, it has minor effect on chlorophenols. On the contrary, at the final stages of photodegradation of chlorophenols, the rates were an order of magnitude faster in presence than in absence of dioxygen. Substitution of WO moiety of SiW₁₂O⁴⁻₄₀ for transition metals, namely, [H₂OMn^IISiW₁₁O₃₉]⁶⁻ and [H₂OCu^IISiW₁₁O₃₉]⁶⁻ reduced, by about an order of magnitude, the effectiveness of the catalyst.