This study investigates the effect of polyvinyl pyrrolidone (PVP) of different molecular weights on the structure and permeation properties of polysulfone (PSf) membranes. The membranes were prepared by phase inversion method using PSf in two solvents, viz. N-methyl-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc) separately. The surface morphology of the resulting membranes was analyzed by scanning electron microscope (SEM) and liquid displacement method. The membranes were subjected to ultrafiltration characterizations such as measurement of pure water flux (PWF), compaction factor (CF), equilibrium water content (EWC), hydraulic resistance and bovine serum albumin (BSA) rejection. Results show that the morphological parameters and flux performance of the membranes have a significant inter-relationship with the molecular weight of PVP. The membrane pore number and pore area are seen to increase with molecular weight of PVP. However, at a constant pressure, the PWF is found to decrease while BSA rejection increases significantly for the membranes prepared with both the solvents. For the membrane composition considered in this study, the solvent DMAc is found to be more suitable than NMP, in terms of BSA rejection, irrespective of the pH of the BSA solution. The maximum rejection found in this study is 76% with PSf/DMAc membrane with PVP 360,000 (at pH 9.3). (c) 2008 Elsevier B.V. All rights reserved.