Membrane fouling is a major obstacle to the full exploitation of the membrane capacity. This study focuses on conditions that lead up to fouling from natural organic matter (NOM) in drinking water treatment, by analysing filtration mechanisms during non-fouling operation. NOM consists of a wide range of particle sizes and therefore several mechanisms of particle migration have been considered. A particle description for NOM has been developed, including an equation for the size distribution. A new mathematical model based on a mass balance for particle migration makes it possible to predict the critical conditions for fouling by NOM. It has been found that particles in the size range 0.1 to 1.5 mum are most critical for fouling, but the size depends on the conditions during filtration. It appeared that fouling starts at much lower concentrations of NOM on the membrane surface than commonly presumed for example from protein fouling. This is because free NOM particles in natural soft water have very low internal density of organic material. Fouling starts at surface concentrations above approximately 5 g/l on membranes that do not suffer from adsorption of NOM. (C) 2004 Elsevier B.V. All rights reserved.