Flat-sheet hemodialysis membranes were prepared by phase inversion technique using polyether sulfone (PES) dissolved in dimethylacetamide (DMAc) with and without the addition of polyvinylpyrrolidone (PVP). The effect of the composition of the casting solution on membrane morphology and performance were investigated. The performances of membranes were elucidated on the basis of removal of uremic toxins (urea, uric acid, and creatinine) from human blood serum. The membrane prepared from 12 wt % PES with 2.8 wt % PVP demonstrated better performance compared to the other compositions. The membrane performance is a consequence of membrane morphology. Membranes with channel-like or long finger-like structures provide superior removal efficiencies. If the morphology turns to a sponge structure, the effectiveness is diminished. (C) 2004 Wiley Periodicals, Inc.