In order to confirm that benzyl alcohol photooxidation is initiated by benzaldehyde and not by impurities which commercial benzaldehyde might contain and to estimate the extent to which autooxidation in benzyl alcohol-benzaldehyde mixture affects the evaluation of the photocatalytic performance of semiconductor catalysts, the following experiment was carried out. Since no benchmark hydrophobic photocatalyst that could be dispersed in organic phase was available, the photocatalytic oxidation of benzyl alcohol was carried out under solventfree monophasic conditions, i.e., in benzyl alcohol only (Figure ), whereby a powder of potassium poly(heptazine imide) (KPHI), an ionic form of polymeric carbon nitride prepared by reported procedure, was used as a photocatalyst as it is well known to be active in photocatalytic H2O2 production. As expected, KPHI showed high activity in benzyl alcohol oxidation with concomitant H2O2 production yielding 0.61 and 0.95 mmols of H2O2 and benzaldehyde, respectively, after 24 h of irradiation (Figure , Table ). Interestingly, in another run, when after 4 h of irradiation the KPHI photocatalyst was removed from the suspension at a moment when only 0.10 mmols of H2O2 was produced, the irradiation was continued and eventually the yield of 0.50 mmols of H2O2 was obtained after 24 h (Table , green curve in Figure ). Notably, this value is rather close to that obtained when carrying out the reaction without filtering the KPHI photocatalyst off (red curve in Figure ). These results clearly show that, in the system comprising the KPHI photocatalyst in benzyl alcohol, the H2O2 production is not only influenced but indeed dominated by the photocatalytic activity of benzaldehyde. Notably, such H2O2 production photocatalyzed by benzaldehyde is not limited to systems with benzyl alcohol as electron donor. We found that ethanol is also a suitable electron and proton donor for this reaction, leading to the formation of 0.35 mmols of H2O2 after 24 h of irradiation under 406 nm LED (Table ). This is in agreement with the study of Bradshaw et al. where it was observed that the presence of ethanol in irradiated benzaldehyde favoured formation of the geminal diol intermediate (Scheme 1, eq. 2).