Long-term experiments at intermediate concentrations of HCl in the aqueous phase (0.5 M) have shown that the chloroalkylsilane-modified PHI materials reduce their activity under prolonged operation times and even the decomposition of previously produced H2O2 can take place (see Supplementary Information, Figure ). In contrast, HMDS-modified PHI remains active even after 72 h of irradiation, steadily forming hydrogen peroxide (Supplementary Information, Figure ). It is evident from the photographic images of the used photocatalysts that KPHI-HTS and KPHI-OTS partially lose their hydrophobic character after long illumination, while this is not the case for KPHI-HMDS that was found to be fully dispersible in the organic layer even after the total of 96 h of photocatalysis (Supplementary Information, Figure ). Nonetheless, at higher concentration of HCl in the aqueous phase, partial hydrophilization of KPHI-HMDS can take place after prolonged irradiation (24 h), as evident from slightly decreased H2O2 production rate (Supplementary Information, Figure ) as well as photographic images of the used material (See Supplementary Information, Figure ). One may conclude that KPHI-HMDS owes its superior photochemical stability to the absence of long alkyl groups that are present in KPHI-HTS or KPHI-OTS and can be susceptible to the reaction with photogenerated holes or reactive oxygen species. Conditions: 5 mL 1-octanol, 5 mL 0.5 M HCl, LED 406 nm (4.2 mWcm -2 ), 22 C, O2 1 atm. The suspensions were saturated with pure O2 before irradiation and after each sampling.