This study investigates the effect of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration. Drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux cannot be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. The rate and extent of BSA fouling were also strongly dependent on the feedwater composition, such as BSA concentration, pH, and ionic strength. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most Severe at the isoelectric point of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the isoelectric point. Increasing ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8. effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient behavior was also affected by the BSA retention of the membrane. (C) 2009 Elsevier B.V. All rights reserved.