Micropollutants in the aquatic environment pose a high risk to both environmental and human health. The photocatalytic degradation of steroid hormones in a flow-through photocatalytic membrane reactor under UV light (365 nm) at environmentally relevant concentrations (50 ng l –1 to 1 mg l –1 ) was examined using a polyethersulfone–titanium dioxide (PES–TiO 2 ) membrane. The TiO 2 nanoparticles (10–30 nm) were immobilized both on the surface and in the nanopores (220 nm) of the membrane. Water quality and operational parameters were evaluated to elucidate the limiting factors in the degradation of steroid hormones. Flow through the photocatalytic membrane increased contact between the micropollutants and ·OH in the pores. Notably, 80% of both oestradiol and oestrone was removed from a 200 ng l –1 feed (at 25 mW cm – 2 and 300 l m – 2 h –1 ). Progesterone and testosterone removal was lower at 44% and 33%, respectively. Increasing the oestradiol concentration to 1 mg l –1 resulted in 20% removal, whereas with a 100 ng l –1 solution, a maximum removal of 94% was achieved at 44 mW cm – 2 and 60 l m – 2 h –1 . The effectiveness of the relatively well-known PES–TiO 2 membrane for micropollutant removal has been demonstrated; this effectiveness is due to the nanoscale size of the membrane, which provides a high surface area and facilitates close contact of the radicals with the very small (0.8 nm) micropollutant at an extremely low, environmentally relevant concentration (100 ng l –1 ).