First, we tested the pure water flux of the virgin PSf and TiO2– GO membranes in a cross-flow membrane filtration system with a windowed membrane cell (Fig. 2(b)). The window allows a tested membrane to be exposed to different light irradiations during a filtration experiment. The initial fluxes of the PSf and TiO2–GO membranes were 120 and 45 L/m2/h (LMH), respectively. As shown in Fig. 9(a), the flux of the virgin PSf membrane in darkness, sunlight, and UV did not change significantly. The slow yet steady flux decline was a result of membrane compaction under hydraulic pressure. For the TiO2–GO membrane (Fig. 9(b)), compared with a slow flux decline (similar to that for the virgin PSf membrane) as observed in the dark experiment, the flux behaviors under sun- light and UV irradiations were dramatically different: a relatively slow flux increase was observed under sunlight while a rapid flux increase was evident under UV and the flux almost tripled after about 4 h of UV irradiation. The slower flux increase under sun- light suggests that sunlight is less effective than UV in increasing membrane hydrophilicity. We hypothesize that the significant increase in membrane flux under UV would most probably be caused by the higher membrane hydrophilicity due to TiO2–GO- enhanced photoactivity. This hypothesis is further strengthened by the observation in Fig. 9(b) that the subsequent flux decreased after the light source was removed and again the flux increased after the UV or sunlight source was turned back on.