The effect of four processing variables on the final properties of ultrafiltration (UF) membranes was investigated. Flat-sheet polyethersulfone (PES) membranes were prepared by wet phase inversion. The variables of interest were: the concentration of base polymer in the casting solution (dope solution), the solvent evaporation time, the addition of surface modifying macromolecules (SMMs), and the use of the additive polyvinylpyrrolidone (PVP). The membranes were characterized by solute transport and scanning electron microscopy (SEM). No appreciable difference between the structures of surface-modified and unmodified membranes was observed by SEM. Membranes prepared with PVP had pure water permeation rates (PWP) significantly higher than membranes prepared without the PVP additive. However, the average polyethylene glycol (PEG 35 ku (35 kDa)) separation by membranes with PVP was approximately 15% lower than the tightest membranes prepared without PVP. The effect of surface modification with fluorinated additives and solvent evaporation time was observed only with high molecular weight cut-off (MWCO) membranes prepared with 12 wt.% PES casting solution. (C) 2004 Elsevier B.V. All rights reserved.