This paper investigated the effect of food to microorganisms (F/M) ratios in the MBRs on membrane fouling propensities at fluxes of 10, 20, and 30L/m2hr (LMH). The high F/M-MBR had different biomass properties, more soluble extracellular polymeric substances (EPS), and faster fouling rate compared to the low F/M-MBR. However, the fouling mechanisms at the three fluxes were dissimilar. At a low flux, the microbial flocs dominant cake layers facilitated catching the soluble EPS to increase resistance. At a high flux, the great accumulation of soluble EPS (especially soluble polysaccharides) to form gel-like cake layers predominantly induced membrane fouling. Supplemental materials are available for this article. Go to the publisher's online edition of Separation Science and Technology to view the free supplemental file.