The molecular weight (MW) distributions of soluble microbial products (SMPs) were measured in membrane bioreactors (MBRs) using membranes with nominal molecular weight cut-offs of 1, 10 and 100 kDa to investigate the impact of each of the fractions, i.e. >100, 10-100, 1-10, and <1 kDa on fouling in MBRs. Total SMPs were measured by summation of carbohydrate and protein SMP. Mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance, as reflected by the modified fouling index (MFI). The results showed that there was no correlation between fractions of >100 kDa and 1-10 kDa and fouling in MBRs. However, there was a strong correlation between fouling and SMP concentration in the range of 10-100 kDa with higher fouling rates associated with higher concentrations of 10-100 kDa fraction.. Statistical analysis also proved that the SMP fraction 10-100 kDa is the most important fraction from a fouling perspective. Furthermore, fouling was found to decrease by increasing the <1 kDa fraction of SMP which was confirmed with the results of statistical analysis indicating a negative correlation between the fraction <1 kDa and the fouling rate. (C) 2010 Elsevier B.V. All rights reserved.