In order to tackle this challenge using PCN-based photocatalysts that are known to be good photocatalyst for H2O2 production, the crucial question to be addressed is: Are we able to synthesize a hydrophobic PCN-based photocatalyst for use in a welldesigned biphasic system? To the best of our knowledge, no synthetic protocols for hydrophobic PCN particles that could be selectively suspended in the organic phase of a biphasic system have been available so far. Kumru et al. and Dong et al. developed various procedures for grafting hydrophobic functions onto carbon nitrides, resulting, however, only in amphiphilic materials. Although conventional melon polymers possessing amphiphilic properties could be further modified with alkyl-and fluoroalkyl moieties, it only resulted in the improved dispersibility of PCN in organic solvents while retaining their amphiphilic nature. We hypothesized that the lower degree of hydrogen bonding, better dispersibility and smaller particle size of PHI-based ionic carbon nitrides that can be achieved upon sonication can make them more suitable for hydrophobization than conventional melon-like PCNs. Herein, we achieve, for the first time, effective hydrophobization of ionic carbon nitride (PHI) nanoparticles in the form of a composite with hydrophobic silica bearing methyl-or alkylsilane functions. The obtained PHI-based photocatalyst is hydrophobic and can be selectively suspended in the organic layer of a biphasic water/organic solvent system. This allows us to demonstrate unambiguously the expected advantages of selective photocatalysis in a biphasic system that enables immediate extraction of the H2O2 product into the aqueous phase, thus avoiding the decomposition of H2O2 on the surface of the hydrophobized photocatalyst. Specifically, we demonstrate highly effective visible light-driven reduction of dioxygen to H2O2 with concomitant oxidation of a model fatty alcohol (1-octanol) in the organic phase, resulting in H2O2 concentrations in the aqueous phase that are unprecedentedly high for photocatalytic H2O2 production systems reported so far (Figure ). Since fatty alcohols are readily available as subproducts of the forestry and pulping industry, the implications of our findings for the development of sustainable and economically viable photocatalytic H2O2 production from low-cost feedstock are discussed. The treatment leads to a partial exchange of surface NH2-groups for ionic =C=N -functions, thus reducing H-bonding in the polymer and allowing for the preparation of colloidally stable dispersions of small KPHI particles. The surface of suspended particles is modified with dispersed silica species via a sol-gel process and further functionalized by silylation, yielding a hydrophobic composite. Under visible light (~406 nm) irradiation of a vigorously stirred biphasic system, the produced composite efficiently photooxidizes fatty alcohols in the organic phase (top layer in the image), while H2O2 produced by O2 photoreduction is immediately extracted into the aqueous phase (bottom layer in the image). Such separation of the product and the photocatalyst enables accumulating high concentrations of H2O2 in the water layer, which is not achievable in case of hydrophilic photocatalysts that can effectively decompose the formed H2O2.