Miscibility of polysulfone (PSf) with poly(1-vinylpyrrolidone-co-styrene) (P(VP-S)) copolymers containing various amount of 1-vinylpyrrolidone (VP) was explored. PSf and P(VP-S) copolymers formed miscible blends when copolymers have VP from 68 to 88 wt.% and beyond these critical limits to form immiscible blends. Membranes for the ultrafiltration process were prepared from PSf blends with P(VP-S) copolymers. The membranes prepared from the miscible blends of PSf and P(VP-S) exhibited higher water flux than the membranes prepared from PSf without any loss in solute rejection. The water flux of the membrane prepared from the PSf/P(VP-S)70/1-methyl-2-pyrrolidone (NMP) = 13/9/78 casting solution was increased up to 18 l/(m(2) h bar) with 92% rejection of polyethyleneglycol (PEG) 20,000. However, the solute rejection examined with the membranes fabricated from the immiscible blends of PSf and P(VP-S) lower than that of PSf membranes. The morphologies of membranes were examined to rationalize the observed membrane performance. The cross-sectional morphology of membrane prepared from the miscible blend is similar to that of the PSf membrane but with a thick sponge-like skin layer while that of membrane prepared from the immiscible blend showed separated phase structure. The relatively large pore in the membrane prepared from the immiscible blend results in low solute rejection. (C) 2003 Elsevier Science B.V. All rights reserved.