Flat sheet asymmetric polymeric membranes were prepared from homogeneous solution of polysulfone (PSf) by phase inversion method. N-methy1-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc) were used as solvents separately. Polyethylene glycol (PEG) of three different molecular weights (400Da, 6000Da and 20000Da, respectively) were used as the polymeric additives in the casting solution. The morphology and structure of the resulting membranes were observed by scanning electron microscope (SEM). The pore number, pore permeability and their distribution and average pore size of the membranes were determined by the liquid displacement method. The permeation performances of the membranes were evaluated in terms of pure water flux (PWF), equilibrium water content (EWC), hydraulic permeability, and solute rejection. Solution of bovine serum albumin (BSA) of molecular weight 68,000 Da was used to study the permeation performance of prepared membranes using a batch membrane cell of 100 mL capacity. Results showed that with increase in molecular weight of PEG, the pore number as well as pore area in membranes increases. Membranes with PEG of higher molecular weights have higher PWF and higher hydraulic permeability due to high porosity. With increase in molecular weight of PEG from 400 to 20000, the PWF increases from 15.3 to 2713.4 L m(-2) h(-1) with NMP as solvent while with DMAc as solvent, the PWF increases from 24.5 to 555.6 L m(-2) h(-1). Similarly, EWC increases from 56.8% for PEG 400 to 78.8% for PEG 20000 for PSf/NMP/PEG membranes. Similar trend is observed for PSf/DMAc/PEG membranes. The BSA rejection data is maximum with PEG 6000 for both the solvents and the values are 56.4% for NMP (at pH 4.8) and 42.4% for DMAc (at pH 9.5). (C) 2007 Elsevier B.V. All rights reserved.