The impact of fouling on N-nitrosamine rejection by nanofiltration (NF) and reverse osmosis (RO) membranes was investigated in this study. Membrane fouling was simulated using tertiary treated effluent and several model fouling solutions (that contained sodium alginate, bovine serum albumin, humic acid or colloidal silica) to elucidate the changes in rejection behaviour of N-nitrosamines. In general, the rejection of N-nitrosamines increased when the membranes were fouled by tertiary effluent. The rejection of small molecular weight N-nitrosamines was most affected by membrane fouling. In particular, the rejection of N-nitrosodimethylamine (NDMA) by the ESPA2 membrane increased from 34 to 73% after membrane fouling caused by tertiary effluent. The results also indicate that the impact was less apparent for the lowest permeability membrane (i.e., ESPAB), and the rejection of N-nitrosamines by the ESPAB membrane was over 82% regardless of membrane fouling. The effect of membrane fouling caused by model foulants on N-nitrosamine rejection was considerably less than that caused by tertiary effluent. Size exclusion chromatography analyses revealed that the tertiary effluent contains a high fraction of low molecular weight (< 500 g/mol) organic substances. It appears that these low molecular weight foulants present in the tertiary effluent can restrict the solute pathway within the active skin layer of membranes, resulting in the observed increase of solute rejection.