Nanofiltration (NF) is used in water and wastewater treatment as well as water recycling applications, treating micropollutants such as hormones. Due to their potential health risk it is critical to develop effective treatment processes. Polymeric NF membranes should be effective in removing such micropollutants based on molecular size. However, the occurrence of adsorption onto the membranes results in unpredictable performance. It is hence important to understand NF retention mechanisms. The focus of this study was to understand how estrone and estradiol adsorption and retention are affected by membrane operational parameters such as pressure. Reynolds numbers (based on channel height Re(h)) and feed concentration for the NF270 membrane. These variables are known to contribute to concentration polarisation, and therefore affect sorption and retention by NF membranes. The total mass adsorbed of both hormones was found to be governed by the initial concentration at the membrane surface. For example, for estradiol the increase of Re(h) number (427-1450) and pressure (3-17 bar) caused the total mass adsorbed to decrease (0.7-0.5 ng cm(-2)) and increase (0.4-0.8 ng cm(-2)), respectively. The same trends were obtained for estrone. Steady-state retention however was found to be dependent on the initial polarisation modulus, given by the ratio between the initial concentration at the membrane surface and the initial feed concentration. For estradiol at the same pressure conditions as above, the polarisation modulus increased from 1.1 to 1.9, causing a decrease of retention from 80% to 51%, whilst for the above Re(h) conditions, the polarisation modulus decreased from 1.5 to 1.0 causing an increase in retention from 69% to 83%. Following on from these results, a model based on a first order sorption kinetics was developed for this membrane allowing the prediction of the transient feed and permeate concentrations for the two hormones estrone and estradiol and a wide range of filtration parameters. (C) 2011 Elsevier B.V. All rights reserved.