Submerged membrane bioreactors (MBRs) were operated at influent protein to carbohydrate (P/C) ratios of 2/1, 4/1, and 8/1, a hydraulic retention time (HRT) of 4.5 h and solids retention time (SRT) of 15 days to determine the impact of P/C ratio on the membrane fouling. Starch and casein were used as the sources of carbohydrate and protein, respectively. Membrane permeability and fouling rates were determined and the protein and carbohydrate content in soluble microbial products (SMP) and extracellular polymeric substances (EPS) were examined. Molecular weight and relative hydrophobicity (RHo) of SMP and EPS were measured as protein (SMPp and EPSp) and carbohydrate (SMPc and EPSc) The mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance expressed as the modified fouling index (MFI). In terms of membrane fouling rate, the MBRs at P/C ratios of 2/1 and 8/1 showed the lowest and highest fouling rates, respectively. The results showed increased concentrations of SMP as both carbohydrate and protein at the highest P/C ratio as well as increased RHo which resulted in pore blocking and increased the fouling rates. Furthermore, the lower EPS concentrations at the higher P/C ratios (4/1 and 8/1) resulted in the formation of smaller flocs which led to higher cake resistance and higher fouling rates. Higher hydrophobic protein EPS concentrations at the higher P/C ratios caused higher attachment of EPS on membrane surface and reduced membrane permeability. (C) 2008 Elsevier B.V. All rights reserved.