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 asmodel foulants at different initial permeatewater fluxes.Water flux declinewas insignificant at an initial permeate flux of 9 L/m2h and the fouling layerwas loose and fluid-like. By contrast, thewater flux declinewas substantial at an initial permeate flux of 20 L/m2h, resulting in the formation of a compact fouling layer. Water flux recovery after physical cleaning for both humic acid and colloidal particle fouledmembraneswas consistently higher at an initial permeate flux of 9 L/m2h compared to 20 L/m2h. The results suggest that the fouling layer structure varied froma fluidlike loose layer at lowinitial permeate flux to amore 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.