This paper investigates the separation process of two estrogenic hormones, estrone and estradiol, using eight commercial NF and low pressure RO membranes. The results indicate that the separation mechanism of estrone and estradiol in membrane filtration processes is similar. While estrogenic hormone retention by more porous membranes decreases with decreasing adsorption and the subsequent retention is relatively low, tighter NF and RO membranes can retain estrogenic hormones effectively. It appears that the presence of organic matter in solution can enhance retention due to the interaction of such substances with estrogenic hormones. The results also suggest that physicochemical interactions within the membrane can play an important role. There is no cross-flow velocity effect on retention, whereas an increase in transmembrane pressure may lead to a decrease in steroid hormones retention for some membranes. (C) 2004 Elsevier B.V. All rights reserved.