Filtration phenomena of the hydrophobic ionogenic compound triclosan by three commercially available nanofiltration (NF)/reverse osmosis (RO) membranes were investigated in this study. The separation behaviour of triclosan was systematically related to the compound physicochemical properties and the membranes as well as the solution pH. The influence of membrane fouling on the rejection of triclosan by the three selected NF/RO membranes were also examined using three model organic foulants and one model colloidal foulants. Having a unique hydrophobic ionogenic profile, even when triclosan was fully deprotonated at pH 10, considerable adsorption of the compound to the membrane could still be observed. The adsorption of triclosan to the membrane surface was a precursor for the diffusion of this compound through the membrane. In fact, under clean (virgin) membrane condition, considerable diffusive transport of triclosan through the loose nanofiltration NF-270 as well as the tight nanofiltration NF-90 membrane was clearly evident, leading to a lower rejection of triclosan by these membranes than expected based on their molecular weight cut-off or the size exclusion mechanism. This diffusive transport appears to be dependent on the thickness of the active layer and its porosity. As a result, no triclosan was detected in the permeate samples from the reverse osmosis BW-30 membrane which has a thicker and more dense active skin layer. Results reported in this study also indicate that the formation of a hydrophobic fouling layer on the membrane surface could interfere with the solute-membrane interaction, and thus, reduce the diffusive transport of triclosan across the membrane. Consequently, significant enhancement in rejection of triclosan was observed when the membranes were pre-fouled with the three model organic foulants namely bovine serum albumin (BSA), alginate and humic acid used in this study. In contrast, no discernible variation in rejection was observed when the membranes were pre-fouled with hydrophilic silica colloids as compared to the clean membrane condition. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.