Ferric ion (Fe[III]) catalyzes the decomposition of hydrogen peroxide (H₂O₂) into strong oxidants such as hydroxyl radical (OH) and ferryl ion (Fe[IV]) through the redox cycling of the iron couple (Fe[II]/Fe[III]). The use of these reactions for the catalytic oxidation of organic compounds is usually limited to the acidic pH region due to the low solubility of Fe(III) and the low efficiency of oxidant production at neutral pH values. The addition of phosphotungstate (PW₁₂O₄₀³⁻), a polyoxometalate, extends the working pH range of the Fe(III)/H₂O₂ system up to pH 8.5. PW₁₂O₄₀³⁻ forms a soluble complex with iron that converts H₂O₂ into oxidants. The coordination of Fe(II) by PW₁₂O₄₀³⁻ also alters the mechanism of the reaction of Fe(II) with H₂O₂ at neutral pH, resulting in formation of an oxidant capable of oxidizing aromatic compounds. The base-catalyzed hydrolysis of PW₁₂O₄₀³⁻ gradually results in inactivation of the catalyst. In the absence of Fe(III), PW₁₂O₄₀³⁻ was completely hydrolyzed after 1 day at pH 7.5, whereas the Fe(III)–PW₁₂O₄₀³⁻ complex was active for at least 4 days under the same conditions.