This study investigated the removal of natural organic matter (NOM) from water, and its effect on membrane fouling, by mixtures of heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) in both batch and microgranular adsorptive filtration (mu GAF) reactors. In batch reactors, increasing the portion of HAOPs in the adsorbent mixture led to a steady increase in membrane fouling. However, this trend was reversed when the water was pretreated with the same adsorbent mixtures in mu GAF reactors. The opposing trends tracked the trend for removal of the biopolymer fraction of the NOM. The results are consistent with a hypothesis that the fouling was caused primarily by formation of a condensed, biopolymer-rich gel layer on the membrane surface. According to this hypothesis, when the feed was pretreated in batch reactors, some foulants were collected on each adsorbent, with PAC collecting more foulant than HAOPs did, but the accumulated material did not condense into a gel. However, when mu GAF reactors were used instead of batch reactors for pretreatment, the tight packing of the adsorbent particles promoted the formation of a gel, leading to increased NOM removal in general and increased foulant removal in particular. The fact that fouling mitigation increased with increasing amounts of HAOPs and decreasing amounts of PAC in the mu GAF adsorbent layer suggests that HAOPs are much more effective than PAC at promoting formation of the gel layer. Generalizing this result, it appears that pretreatment by mixtures of adsorbents in mu GAF reactors could have significant advantages over conventional approaches in which a single adsorbent is applied in a well-mixed reactor.