In order to investigate the photochemical reactivity of biphasic systems of benzyl alcohol/benzaldehyde and water, we first irradiated a mixture composed of equal volumes of benzyl alcohol and H2O under UV light (365 nm LED; Figure ). Unlike the case of Pavan et al. who worked with benzyl alcohol solutions in acetonitrile, we did not observe any initiation period of benzyl alcohol photochemical conversion and H2O2 formation in the aqueous phase (Figure ). The irradiation of this biphasic system for 24 h yielded 2.0 mmols of H2O2, which corresponds to the concentration of H2O2 in the aqueous phase of 0.4 M, a very high value that is far beyond the values typically achieved in light-driven H2O2 production! The 1 H NMR analysis of the organic phase confirmed the presence of benzaldehyde by the resonance at 9.9 ppm and formation of benzoic acid is also suggested by new peaks appearing in the NMR spectrum, in particular one at 8.2 ppm (Figure ). We conclude that the reaction is most likely initiated by benzaldehyde from self-oxidation of benzyl alcohol, and proceeds via benzaldehyde-mediated auto-photocatalytic mechanism shown in Scheme 1. In accord with the previous works on benzyl alcohol photooxidation, only small amounts of H2O2 were produced by irradiation of benzyl alcohol under visible light only (406 nm LED) (Figure ). The H2O2 production was slightly intensified in the biphasic system containing hydrochloric acid (Figure ), which was likely due to a higher stability of H2O2 under low pH or to its facilitated formation by reaction of the protons with reactive oxygen species. Nevertheless, such experiments in which only the substrate is irradiated to rule out any autooxidation effects in a photocatalytic system might be deceiving since they do not take into account possible photochemical activity of intermediates formed upon substrate oxidation.