Water splitting via two two-electron processes (the H₂O first photocatalytically converted to H₂ and H₂O₂ under visible light irradiation and then the H₂O₂ disproportionation to H₂O and O₂ by a thermal catalytic process) has attracted extensive attention recently.1,2 Contrary to these reports, we found that not only the photocatalytic H₂ generation could be driven by visible light but also the two-electron H₂O₂ disproportionation to form H₂O and O₂ could also be photocatalyzed by visible light over g-C₃N₄ catalysts. Photocatalytic H₂, O₂ generation, and simultaneous H₂O₂ formation in Cu/C₃N₄ and Fe/C₃N₄ dispersions were confirmed, about 2.1 and 1.4 μmol of H₂ and 0.8 and 0.5 μmol of O₂ evolved over Cu/C₃N₄ and Fe/C₃N₄ in 12 h, respectively. To prove the photocatalytic process of H₂O₂ disproportionation, the H₂O₂ was added as a reagent in g-C₃N₄, Cu/C₃N₄, and Fe/C₃N₄ dispersions. The results showed that the activity of H₂ evolution decreased with the increase of H₂O₂ concentration; the corresponding AQEs of oxygen formation were 16.1%, 42.6%, and 78.5% at 400 nm, respectively. The remarkable increase of anodic photocurrents over Fe/C₃N₄/ITO and Cu/C₃N₄/ITO electrodes indicated that the two-electron H₂O₂ disproportionation was catalyzed via surface photocatalytic mechanism. The ESR results implied reaction occurred by O₂^–· radical path over g-C₃N₄ under irradiation.