In order to further corroborate our conclusions, we tested the tendency of the photocatalysts towards decomposition of H2O2 under optimal biphasic conditions with H2O2 added to the reaction system from the start. The hydrophilic photocatalyst KPHI and KPHI-Si after 24 h of illumination cause the reduction of initially introduced H2O2 concentration from 91.8 mM to 35.2 mM and 47.6 mM, respectively (Supplementary Information, Figure ). These values are in the same range as those observed during the H2O2 evolution in biphasic system for the same photocatalysts in the interval from 24 h to 72 h of irradiation (Figure ). In contrast, when H2O2 is added at the start of the reaction using the hydrophobic KPHI-HMDS photocatalyst, it continues to produce H2O2 resulting in its higher concentration after 24 h of reaction compared to the initial one (Supplementary Information, Figure ). These results are in line with changes of the apparent quantum yield (AQY) of H2O2 formation in the course of the photocatalytic reaction under biphasic conditions (Figure , see Supplementary Information 2.4 on the details of estimation and Figure for comparison under monophasic conditions) which shows that the differences in the performance of the hydrophilic and hydrophobic photocatalysts are more evident after prolonged irradiation times. The AQY of the best-performing hydrophilic photocatalyst drops drastically from 7.6% after 1 hour to 2.9% after 24 hours, i.e., by a factor of ~2.5. In contrast, the champion hydrophobic PHI-based composite achieves the AQY values of 9.4% and 5.7% after 1 h and 24 h, respectively, i.e., a drop by the factor of only ~1.6, hence after 24 hours the AQY is nearly twice as high for the hydrophobized photocatalyst than for the hydrophilic one. All these results confirm the highly beneficial effect of using a hydrophobized photocatalyst in a biphasic system for light-driven H2O2 production. In addition, we also confirmed that the optimal photocatalyst loadings established on the basis of the experiments carried out in monophasic conditions are applicable to the biphasic system as well. The increase of the KPHI-HMDS photocatalyst concentration in the biphasic system yields only moderate improvement of the H2O2 production (>0.12 M), while the doubling of the KPHI-Si loading has an opposite effect leading to a worse performance (see Supplementary Information, Figure ).