Polymeric flat-sheet membranes with different properties were used in filtration experiments with activated sludge from a pilot-scale MBR to investigate the influence of membrane pore size, surface porosity, pore morphology, and hydrophobicity on membrane fouling. An improved flux-step method was used to measure both the critical flux and critical flux for irreversibility. Long term experiments were performed to evaluate if influences of membrane properties on short term could be translated to long term fouling behavior. The results showed that a hydrophilic asymmetric membrane with an interconnected pore structure, a nominal pore size of 0.3 mu m, and large surface porosity of 27%, provided the best membrane performance with respect to critical flux and critical flux for irreversibility. The dominant fouling mechanism in long term filtration experiments was gel layer formation, which for this membrane was the least severe, and therefore extended the sustainable time. (C) 2009 Elsevier B.V. All rights reserved.