The phenomenon of concentration polarization and gel formation plays an important role in ultrafiltration of macromolecules from water. However, there is lack of studies on similar phenomena for macromolecule ultrafiltration from blends of aqueous and non-aqueous solvents. This paper reports the effect of ethanol concentration on flux in ultrafiltration of polyethylene glycol and dextran with a solvent resistant polymeric membrane. Transient filtration data, collected after steady state, was utilized to determine actual filtration resistance and contribution of polarization using the osmotic pressure model. It was observed that both steady state flux and gel formation were significantly influenced in the ultrafiltration of both PEG and dextran from blended solvents. It was also observed that the onset of gel formation for PEG and concentration polarization for dextran in blended solvents occurred at significantly higher bulk solute concentrations than in water alone. (C) 2004 Elsevier B.V. All rights reserved.