This paper examined the effect of aeration on the quantity and composition of microbial polymers in sludge attached on the membranes as well as in mixed liquor and the consequences for membrane fouling in submerged membrane bioreactors (MBRs). Three lab-scale submerged MBRs with aeration rates of 120, 80 and 40 l/h were operated for 170 days at a constant permeate flux of 8 l/(m 2 h) with hollow fiber membranes. The composition of sludge attached on the membranes was found to depend on the composite action of corresponding composition in mixed liquor and membrane hydrophobicity. The protein/carbohydrate ratios of sludge attached on the membranes were observed to increase with operation cycle number, which was probably due to the preferential adsorption of solutes and sludge particles with higher protein/carbohydrate ratios on the membranes. There was a positive correlation between the final transmembrane pressure (TMP) at the end of a cycle and the product of protein/carbohydrate in sludge flocs attached to the membranes and V f (VSS mass on unit membrane area at the end of each cycle). Furthermore, total polymers in sludge flocs appeared more important than EPS in reflecting the extent of membrane fouling. Finally, based on the above conclusions, a new explanation for the influence of aeration on membrane fouling was presented.