Nanofiltration membranes should be effective in removing hormones based on hormone molecular size. However, the occurrence of adsorption onto the membranes results in a lower performance than would be expected by size exclusion. It is hence important to understand the retention mechanisms involved in the removal of adsorbing trace contaminants. The focus of this study was to elucidate how estrone and estradiol adsorption and retention are affected by intrinsic membrane characteristics such as different polymeric materials and membrane pore radius. Polyamide raw material and polyamide active layer of TFC NF membranes were found to adsorb much higher amounts of hormones than any of the other membrane materials that constitute the membranes, i.e., polysulfone and polyester. These results show that the bulk of the adsorption occurs in the active layer. The adsorption isotherm onto the different raw polymeric materials was found to be of the Freundlich type, and interactions between hormones and the different polymers can be explained by H-bonding and weak pi-pi interactions, amongst other interactions, and not hydrophobic interactions. Adsorption and retention were further found to be affected by the membrane active layer pore size, hence the steric exclusion capacity of the membrane, which dictate show much hormone partitions into the membrane pores. An increase of pore radius from 0.32 nm to 0.52 nm increased the amount of hormone that partitions into the membrane pores, thus affecting adsorption, which increased from 0.17 ng cm(-2) to 1.10 ng cm(-2). Retention, on the other hand, decreased from 88% to 34%. Finally, hormones were shown to penetrate and adsorb inside the active layer at pH 7, whilst at pH 11, adsorption was confined to the membrane surface due to electrostatic repulsion. The membrane internal surface area of the active layer played a role in adsorption. At neutral pH, the more internal surface area the membrane had, the more adsorption took place. There is therefore a combination of partitioning effect and internal surface area access playing a role in hormone adsorption and retention by NF membranes. (C) 2012 Elsevier B.V. All rights reserved.