Nevertheless, the profiles of the H2O2 evolution reported by Yamashita et al. clearly suggest that an autocatalytic mechanism might be at play since the H2O2 production was accelerated after certain irradiation time. This prompted us to investigate the issue of benzaldehydemediated auto-photocatalysis in H2O2 production systems in detail. Herein, we provide a conclusive evidence for the ability of benzaldehyde to drive auto-photocatalytic oxidation of benzyl alcohol and produce large quantities of H2O2 in a solvent-free (no water) or biphasic (with water) system under nominally visible light irradiation (>420 nm cutoff filter) even in the absence of any heterogeneous photocatalyst. We discuss the impact of our findings on the reliability of interpretation of photocatalytic H2O2 production in studies in which benzyl alcohol was employed as model electron donor, and provide simple check protocols for excluding similar issues when using alternative substrates. Scheme S1. Proposed mechanism of auto-photocatalytic production of H2O2 from benzyl alcohol and dioxygen using benzaldehyde as a photocatalyst. Benzaldehyde is excited by light irradiation to a triplet excited state (1). The excited state of benzaldehyde abstracts hydrogen atom from benzyl alcohol under formation of a diol (2). The diol reacts with dioxygen to yield -hydroxybenzylhydroperoxide and benzaldehyde (3). The -hydroxybenzylhydroperoxide intermediate decomposes rapidly to H2O2 and benzaldehyde (4). Note that i) since benzaldehyde is the light-absorbing species and is not consumed during the reaction cycle, the conversion is truly photocatalytic; ii) since the product of benzyl alcohol oxidation is benzaldehyde, i.e. the photocatalyst, the reaction is also autocatalytic.