Fouling of low-pressure membranes treating natural waters can be substantially mitigated by pre-depositing a thin layer of micron-size adsorbent particles on the membrane, a process we refer to as microgranular adsorptive filtration (mu GAF). The role of adsorbent particle size, adsorbent surface loading, and membrane pore size in fouling of mu GAF systems by natural surface water has been investigated. mu GAF tests using heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) reveal that fouling in such systems occurs both on the membrane and in the cake layer. Fouling on the membrane is primarily caused by soluble NOM and is exacerbated by the use of larger adsorbent particles and smaller-pore membranes. Such fouling is mitigated by removal of foulants in the pre-deposited layers, so the extent of mitigation is proportional to the adsorbent surface loading (i.e., the thickness of the cake layer). By contrast, fouling in the cake layer is caused by larger foulants such as colloids and particulate matter. Such fouling is insensitive to the layer's thickness. Use of smaller adsorbent particles improves the capture of colloids and particles but also exacerbates such fouling. In these cases, increasing the membrane pore size decreases the rate of fouling on the membrane, but does not affect the cake layer fouling. (c) 2013 Elsevier B.V. All rights reserved.