Fouling of an ultrafiltration membrane by raw lake water, the same water after passage through a layer of heated aluminum oxide particles (HAOPs), and the fractionated natural organic matter (NOM) (based on hydrophobicity) of each of those waters was investigated. When each fraction of the raw water was fed to the membrane at the same DOC concentration, the hydrophobic (HPO) fraction was the least well retained by the membrane, but it caused the most fouling, whereas the hydrophilic (HPI) fraction was the most efficiently retained fraction and caused the least fouling. Furthermore, whenever the feed contained mixtures of different fractions, fouling was worse than would be predicted by a simple mixing model, indicating that interactions among the fractions are important factors in the fouling process. Treatment of the raw water by HAOPs substantially removed fulvics from all five feed solutions (raw water, the HPO, TPI [transphilic], and HPI fractions, and the reconstituted water) and humics from all except the HPI fraction (which contained very little humic material in the first place). In contrast, HAOPs treatment removed aromatic proteins only from the HPI fraction. HAOPs also selectively removed higher MW NOM. The treatment significantly mitigated membrane fouling by all the fractions to a greater extent than would be expected based on DOC removal. The fouling of membranes fed either raw water or HAOPs-pretreated water was largely reversed by alkaline cleaning. The NOM that was released by the cleaning step was enriched in aromatic proteins and non-fluorescent fractions, but it contained much less humic and fulvic matter and overall fluorescence than raw water. The results suggest that a non-fluorescent HPO fraction of the NOM accumulated on the membrane and is responsible for substantial fouling, and that interactions among different fractions are important contributors to fouling. (C) 2016 Elsevier B.V. All rights reserved.