The dynamics of fouling of a microfilter by natural organic matter (NOM) were investigated in serial filtration experiments. Only a small fraction (<3%) of the NOM molecules was rejected by the membrane, but the rejected NOM significantly reduced the permeate flux through the 0.025 mu m membrane pores. Fouling rates increased with increasing flux. Fouling occurred initially by narrowing of the pore openings, which eventually became completely covered by an NOM gel. No gel was found inside the pores. The fouling rate in each run increased significantly when the pore openings became completely covered with gel, but permeation through the gel continued. The NOM gel was compressible, but the compression had a significant effect on the overall system resistance only after the gel layer covered the whole membrane surface. NOM molecules in the permeate from the first filtration step caused almost no fouling if they were immediately passed through a second, identical filter. However, the longer the permeate was stored before the second filtration step, the more severely it fouled the second membrane. SEM images of the downstream membrane showed that fouling occurred both by pore narrowing by NOM gel and by pore blockage by aggregates of distinct NOM globules. It is proposed that the NOM exists in the raw water in units with a pseudo-equilibrium size distribution. Some of these units collect on the membrane and serve as nuclei for accretion and coalescence with other units. Immediately after passage through the membrane, the permeate is depleted in larger NOM units, but if the permeate is stored, a size distribution more like that in the unfiltered water is eventually re-established. Nevertheless, at least for the samples studied here, fouling of the membrane in the second filtration step progressed much less rapidly than in the first step, indicating that the NOM units responsible for fouling were selectively removed. (C) 2007 Elsevier B.V. All rights reserved.