The characteristics of membrane fouling were compared in submerged membrane bioreactors at three Mg:Ca molar ratios (1:5, 1:1, and 5:1), constant calcium concentration of around 1.7mM and a solids retention time (SRT) of 15 d. Permeability, extracellular polymeric substances (EPS), soluble microbial products (SMP), Ca2+ and Mg2+ concentrations were measured regularly in the reactors to investigate the cause of membrane fouling. Molecular weight fractionation and relative hydrophobicity of EPS and SMP were also measured. In terms of membrane fouling rate, no differences were observed between Mg:Ca ratios of 1:5 and 1:1. The increase in the EPS and the concomitant decrease in SMP concentrations, both as protein and carbohydrates, due to divalent cation binding and enhanced bioflocculation were not significant enough to change the membrane permeability. More hydrophobic EPS was formed due to lower divalent cations at the low Mg:Ca ratio, resulting in higher EPS deposition on the membrane surface and membrane fouling. Comparison of the reactors with Mg:Ca ratios of 5:1 and 1:5 showed that the fouling rate is lower in the reactor with the higher Mg:Ca ratio. The introduction of magnesium at the higher Mg:Ca ratios proved that magnesium is beneficial in reducing membrane fouling due to changes in EPS and SMP characteristics as well as molecular weights. (C) 2009 Elsevier B.V. All rights reserved.