In membrane bioreactors, mixed liquor properties influence membrane fouling significantly. An effective method was therefore proposed to identify the major factors that affect membrane fouling under identical conditions for direct comparison. First, the membrane filterability of the mixed liquor was measured to evaluate fouling propensity. Forty mixed liquor samples taken from various membrane bioreactors were measured in terms of their membrane filterability and 14 other properties. The correlation of membrane filterability with each mixed liquor property was further analyzed by multivariate statistical analyses. The results showed that supernatant organic concentration was the most significant factor influencing membrane filterability. The soluble organics and polysaccharides in the supernatant contributed to fouling more than the colloidal organics and proteins. Additionally, mixed liquor suspended solid concentration had an effect on filterability at concentrations greater than 10g L-1, which resulted in a great increase in the viscosity of the mixed liquor. Otherwise, mixed liquor with mean floc size greater than 80 mu m consistently featured good membrane filterability. Extra-cellular polymeric substance content had little effect on membrane filterability. (c) 2009 Elsevier B.V. All rights reserved.