In this study, we evaluated removal of human adenovirus 40 by hollow fiber ultrafiltration (UF, d(pore) = 0.04 mu m) and microfiltration (MF1, d(pore) = 0.22 mu m; MF2, d(pore) = 0.45 mu m) membranes in the presence of suspended and dissolved foulants: SiO2 microspheres and Aldrich humic acid (HA). Average removal of adenovirus from DI water by UF, MF1 and MF2 membranes was 2.3 log, 0.7 log and 0.7 log, respectively. The observed decrease in adenovirus removal due to SiO2 fouling (delta LRV of -12 and -0.2 for UF and MF1 respectively) was attributed to the cake-enhanced accumulation of viruses at the membrane surface. In contrast, fouling by HA led to higher virus removals (delta LRV of 0.8 and 1.2 for UF and MF1, respectively), which was attributed to pore blockage by HA. In experiments with MF2 membrane, neither HA nor SiO2 had significant effects on adenovirus removal. The results indicate that the extent of fouling is not a reliable predictor of adenovirus removal. Instead, feed water composition and membrane pore size together govern virus removal with fouling mechanisms playing a mediating role: pore blockage improves removal while cake formation can either increase or decrease removal depending on cake properties. (C) 2015 Elsevier B.V. All rights reserved.