The photooxidation of pure benzaldehyde immediately yields an insoluble precipitate in the vial, which, according to C NMR analysis, is benzoic acid. Almost quantitative conversion of benzaldehyde to benzoic acid can be observed after 24 h of irradiation similar to the case reported by Mazzanti et al. (Figure , Table ). The presented data unambiguously show that benzaldehyde-mediated photooxidation of benzyl alcohol and concomitant H2O2 production must be accounted for in studies of lightdriven H2O2 production using heterogeneous photocatalysis and benzyl alcohol as electron donor, even under nominally visible light (using a 420 nm cut-off filter). The observed effects might be negligible in very dilute solutions of benzyl alcohol, but they might represent a dominant mechanism under solvent-free (only benzyl alcohol) or biphasic (e.g., equivolumetric benzyl alcohol/water mixtures) conditions. Two further points are noteworthy with respect to the visible light activity. Firstly, the designation of a 420 nm cut-off filter implies, per definitionem, that the transmittance at 420 nm is 50%, i.e., there can be a small portion of higher energy light still transmitted when using these filters. Therefore, it makes sense, as we do, to speak about experiments carried out under nominally visible light irradiation when using a 420 nm cut-off filter. Secondly, pure benzaldehyde does absorb significant portion of visible light, and its absorption tail extends down to 420-430 nm range (Figure ). Moreover, the absorption edge of benzaldehyde apparently depends on the choice of solvent. As an example, one can see a red shift of benzaldehyde absorption edge if it is dissolved in ethanol compared to that dissolved in acetonitrile (Figure ). The solvent-benzaldehyde interaction can affect the clustering of benzaldehyde molecules, disrupting the - interactions in the system and hence influencing the electronic excitation of benzaldehyde. It is likely that especially under solvent-free and biphasic conditions, the - interactions are beneficial for photoexcitation of benzaldehyde at longer wavelengths, leading to the formation of excited states that can react with benzyl alcohol, and resulting in simultaneous benzaldehyde and H2O2 formation according to the mechanism depicted in Scheme 1.