Membrane fouling by natural organic matter (NOM) was investigated in microgranular adsorptive filtration (μGAF) systems, in which a thin layer of adsorbent is predeposited on low-pressure membranes. The adsorbents tested included heated aluminum oxide particles (HAOPs), ion exchange (IX) resin, and powdered activated carbon (PAC). Size exclusion chromatography (SEC) separated the NOM into four apparent MW fractions with significant UV₂₅₄. HAOPs and the IX resin performed almost identically with respect to removal of these fractions, and differently from PAC. However, while HAOPs and PAC reduced fouling substantially, IX resin did not, indicating that fouling could not be attributed to the NOM fractions detected by SEC. Rather, the key foulants appear to comprise a very small fraction of the NOM with almost no UV₂₅₄ absorbance. Alginate, a strongly fouling surrogate for natural polysaccharides, is adsorbed effectively by HAOPs, but not by IX resin or PAC, suggesting that polysaccharides sometimes play a key role in membrane fouling by NOM.