As next we turn to a direct comparison of the photocatalytic H2O2 production performance of the hydrophilic and hydrophobic photocatalysts under biphasic conditions. During the first 4 h of irradiation all hydrophobic photocatalysts show only somewhat higher rates of H2O2 production compared to the hydrophilic ones (Figure , Figure , Table ). However, this changes dramatically after 24 h of the reaction, when only a moderate increase of H2O2 concentration is observed for the systems containing the hydrophilic KPHI and KPHI-Si photocatalysts (Figure ). In this context it should be noted that the observed breakdown in H2O2 production rate using the hydrophilic photocatalysts is not due to the photocatalyst deactivation as the replacement of the aqueous phase with a fresh 0.5 M HCl solution after 72 h of the reaction results in a complete regain of the initial activity. The observed behaviour of the hydrophilic photocatalysts in biphasic system is therefore suggestive of the equilibrium reached in the aqueous system between the formation of new H2O2 via O2 reduction and the decomposition of the already formed H2O2, which clearly limits the maximum achievable H2O2 concentration in the aqueous phase. In contrast, the hydrophobic KPHI-HMDS material does not show any indication of such limitations for at least 96 h of total illumination (Figure ). Moreover, the H2O2 concentration in the separated aqueous phase achieved during photocatalysis with KPHI-HMDS after 72 h irradiation is ~0.12 M. To the best of our knowledge, such a high value has never been reported for any other light-driven H2O2 production systems in which the obtained H2O2 would be already separated from electron donor as well as from the photocatalyst. This impressively demonstrates the advantage of performing the conversion using a hydrophobized photocatalyst in a biphasic system. It is noteworthy that after continuous 24 h irradiation of the KPHI-HMDS under the reaction conditions, it does not lose its hydrophobicity. The recovered material is clearly hydrophobic with a contact angle of 1304 (Supplementary Information, Figure ).