Fouling behaviour and its impact on the rejection of trace organic contaminants (TrOCs) by forward osmosis (FO) were investigated. Membrane fouling was simulated using humic acid and colloidal particles as model foulants at different initial permeate water fluxes. Water flux decline was insignificant at an initial permeate flux of 9 L/m(2)h and the fouling layer was loose and fluid-like. By contrast, the water flux decline was substantial at an initial permeate flux of 20 L/m(2)h, resulting in the formation of a compact fouling layer. Water flux recovery after physical cleaning for both humic acid and colloidal particle fouled membranes was consistently higher at an initial permeate flux of 9 L/m(2)h compared to 20 L/m(2)h. The results suggest that the fouling layer structure varied from a fluid-like loose layer at low initial permeate flux to a more cohesive and compact layer at high initial permeate flux. We surmise that the fluid-like loose layer formed at low initial permeate flux contributed to pore blockage and thus enhanced steric hindrance, thereby leading to an increase in TrOC rejection. By contrast, the cohesive and compact fouling layer formed at high initial permeate flux exacerbated cake-enhanced concentration polarisation, resulting in a decrease in TrOC rejection. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.