The generation of hydroxyl radical (•OH) by visible light-illuminated tungsten oxide (WO₃) was found to be significantly improved in the presence of hydrogen peroxide (H₂O₂) and bicarbonate ion (HCO₃⁻). A ternary system of hν/WO₃/H₂O₂/HCO₃⁻ showed synergistic enhancement in oxidation of benzoic acid (BA, a •OH probe compound) into hydroxybenzoic acids (HBAs), exhibiting even less consumption of H₂O₂ than hν/WO₃/H₂O₂. Analyses of HBAs from BA oxidation (three HBA isomers and 18O-labelled HBA from H₂¹⁸O₂) suggested that hν/WO₃/H₂O₂/HCO₃⁻, contrary to hν/WO₃/H₂O₂, generated •OH mainly via one-electron transfer from the conduction band of WO₃ to H₂O₂. The dominant one-electron reduction of H2O2 over HCO₃⁻-treated WO₃ was further evidenced by Koutecký–Levich plots obtained with a rotating disk electrode setup. Based on different experiments using electron paramagnetic resonance spectroscopy, radical scavengers and probes, (photo-)electrochemical measurements, and density functional theory calculations, the mechanisms underlying the enhanced generation of •OH by hν/WO₃/H₂O₂/HCO₃⁻ were discussed.