Adsorption, desorption, and steady-state rejection of a model estrogenic hormone, 17 beta-estradiol (E2), by polyamide thin-film composite nanofiltration (NF) membranes were investigated. The removal of E2 was enhanced by adsorption to the membrane during the initial phase of filtration. Desorption of adsorbed E2 to permeate resulted in negative apparent rejection when the membrane phase E2 concentration was greater than the equilibrium permeate concentration. Changes in permeate concentration during this unsteady-state filtration period resembled break-through curves of commonly used adsorption media columns. Batch isotherm and filtration experiments performed using intermediate stage products (e.g., membrane without polyamide film coat) suggested that these adsorption/desorption processes involving E2 occurred predominantly at the polysulfone support layer, not at the polyamide active layer, most likely through hydrophobic interaction. Operating conditions such as pressure and water quality parameters such as pH and organic foulants were found to strongly influence the sorption of E2. (C) 2008 Elsevier B.V. All rights reserved.