The influence of MF pretreatment on NF performance was Studied for synthetic solutions of fluorescent monodisperse latex particles of 0.1 and 1 mum diameter and an organic foulant (benzyl alcohol) in water. Samples were microfiltered in the dead-end mode using a membrane with a pore size of 0.22 mum, and subsequently nanofiltered using five fiat sheet nanofiltration membranes in parallel: UTC-20 (Toray Ind. Inc.), Desal 5 DL and Desal 51 HL (Osmonics), NF-PES-10 and N30F (Nadir). Both the particles and the organic foulant were found to cause flux decline in NF. The extent of fouling depended on the NF membrane used; hydrophobic materials were found to cause more flux decline by particles and the organic foulant. Particle fouling was visualized using SEM images of the NF membranes after filtration of the 0.1 mum as well as the 1 mum particles by MF membranes. Smaller particles led to higher flux decline in MF due to more compact cake formation and hi her specific cake resistance in the dead-end mode; 0.1 mum particles caused more internal fouling in the membrane than 1 mum particles. However, without MF pretreatment. the flux decline in NF was more severe for Solutions containing the larger particles (1 mum) than the smaller particles (0.1 mum). due to slower back diffusion from the membrane surface to the bulk solution. An additional influence of the NF spacer material was observed, due to the formation of a local dead-end volume between the fine meshes of the spacer net. When a microfiltration pretreatment was applied, flux decline was limited to the effect of organic fouling of the membrane. Thus, MF pretreatment is effective in avoiding a large fraction of the fouling, in NF, although an effect of organic fouling may remain. Furthermore, a lower rejection of benzyl alcohol in NF was found after MF pretreatment than for NF without pretreatment. (C) 2003 Elsevier B.V. All rights reserved.