A systematic investigation on the influence of colloidal fouling on removal of trace organics (steroid hormones) and inert organics by reverse osmosis (RO) membranes is reported. Results of laboratory-scale crossflow membrane filtration experiments showed that colloidal fouling caused a marked decrease in rejection of inert organic molecules with molecular weight smaller than about 100 g/mol. While the removal of inert organics was mainly due to steric (size) exclusion, adsorption and diffusion across the membrane polymer played a prominent role in the removal of the steroid hormones by the RO membrane. During membrane fouling, rejection of the inert organics and salt declined to a minimum, but then improved slowly and eventually stabilized at a fixed value. Hormone rejection, on the other hand, decreased rapidly initially, and then gradually slowed down regardless of fouling. This rejection behavior is associated with the decrease in feed hormone concentration due to adsorption and the increase in permeate concentration due to diffusion across the membrane skin layer. The decline in hormone rejection with time was much more severe when colloidal fouling took place. The solute rejection behavior is markedly influenced by the cake-enhanced concentration polarization, whereby the colloidal cake layer hinders back diffusion of solutes from the membrane surface to the bulk solution. The buildup of solutes at the membrane surface results in a higher solute concentration gradient across the membrane and, thus, a greater solute transport through the membrane and a lower observed solute rejection. The rate of decline in permeate flux and solute (hormones, inert organics, and salt) rejection correlated inversely with channel wall shear rate. (C) 2004 Elsevier B.V. All rights reserved.