Naturally occurring organic compounds tend to form complexes with divalent cations and micro-pollutants, and to foul membrane surfaces: both phenomena have a significant effect on pollutant rejection. Previous study results show the significant influence of triazine herbicides complexation, with dissolved humic substances, on their rejection by NF/ULPRO membranes. The net effect of fouled membranes, on triazine retention, is systematically investigated herewith by comparing the performance of three types of clean and fouled membranes and relating them to changes observed in membrane surface characteristics. Two typical triazines (atrazine and prometryn) and three well characterized humic substances (HS) are employed. The results show that humic substances deposited on the membrane surfaces cause considerable changes of their characteristics, including the contact angle and salt retention, which affect water permeability and triazine retention. Specifically, a strong correlation is identified between the hydrophobicity/hydrophilicity of dissolved HS and the resistance to flow of the fouling layer, which affects the retention of the smaller-size triazines. This trend is related to the condition of organic layers on the membrane. Generally, relatively loose fouling layers on the membranes are associated with reduced triazine retention. However, rather dense fouling layers formed by complexes of HS with calcium exhibit significant flux decline and an improved sieving effect on triazines. Moreover, tight and hydrophobic membranes display significant changes of triazine retention when fouled by HS of increased hydrophobicity. On the contrary, porous and hydrophilic membranes display significant changes of triazine retention only when fouled by HS of reduced hydrophobicity. The new results highlight the need for good knowledge of the properties characterizing the organic matter present in natural waters, before their treatment process is designed. (C) 2011 Elsevier B.V. All rights reserved.