Hence, the same experiments were performed in the presence of 2 v/v% of benzaldehyde which is the main product of benzyl alcohol oxidation. The formation of H2O2 was observed immediately after starting the irradiation, and its concentration steadily increased throughout 24 h of irradiation, eventually yielding high amounts of H2O2 (0.54 mmols) (Figure ). Again, the acidic pH of the aqueous phase facilitates the H2O2 formation (Figure ), which is in agreement with what was observed by Yamashita et al. Notably, the 1 H NMR analysis of the organic phase after 24 h of irradiation shows that benzaldehyde was not consumed during the irradiation of the solution, but its concentration in the system even increased (Table , Figure ), confirming that it behaves as a molecular photocatalyst (Scheme 1). Surprisingly, the photoconversion of the benzyl alcohol -benzaldehyde system also took place even when using a 420 nm cut-off filter under simulated solar irradiation (~1 sun), hence nominally under visible light (Figure ). Although the produced amounts of H2O2 were smaller in this case than under 406 nm LED light, the concentration of H2O2 reached 11 mM and 24 mM after 4 h and 24 h of irradiation, respectively. These values are comparable to those reported by Yamashita et al. for the biphasic H2O2 production via benzyl alcohol oxidation under >420 nm light. Clearly, the presence of benzaldehyde in the system is responsible for photochemical production of H2O2 even under visible light irradiation. In a control experiment, the irradiation of benzaldehyde-water biphasic mixture (without benzyl alcohol) did not result in high concentrations of H2O2 in the aqueous phase, instead after short time of irradiation an insoluble precipitate, most likely benzoic acid, was formed (Table , Figure ). H2O or 0.1M HCl (5 mL), LED 406 nm (4.2 mW cm -2 ), O2 1 atm, temperature 20 C. For the orange curve the conditions are BA (4.9 mL), BAL (0.1 mL), 0.1M HCl (5 mL), vigorous stirring, O2 1 atm., 1 sun irradiation, >420 nm cut-off filter.