A mathematical model was developed for predicting the membrane bio-fouling potential as the operation conditions in the submerged membrane bioreactor (SMBR). Mass balance equations were computed, in terms of biological kinetics, for the production and degradation of extracellular polymeric substances (EPS) and soluble microbial products (SMP). The modified fouling index (MFI) was used for the prediction of bio-fouling potentials divided by soluble materials (MFISol) and suspended solids (MFISS) at the steady-state. The effect of changes in the solids retention time (SRT) on the bio-fouling potentials was more sensitive than that of the hydraulic retention time (HRT). The MFISol and MFISS were found to increase and decrease, respectively, with increases in the SRT. The simulation results showed that an increased sludge age could decrease the microbial activity in the MBR. The MFISol was slightly decreased with increase in the HRT, which was due to the reduction of the concentrations of SMP. Generally, the contribution of MFISS to bio-fouling was relatively low compared to that of MFISol under given conditions. The simulation results were calculated using the given assumptions and limited conditions only. It was suggested that consideration of the parameter changes caused by the properties of biological flocs, influent, membrane and microorganism community would be required to better predict the bio-fouling potentials in the SMBR. (c) 2006 Elsevier B.V. All rights reserved.