The miscibility of polyethersulfone (PES) with hydrophilic copolymers containing 1-vinylpyrrolidone and styrene as repeated units was explored. Among these blends, it was found that PES formed homogeneous mixtures with poly(1-vinylpyrrolidone-co-styrene) copolymers (P(VP-S)) containing VP from 59 to 92 wt.%. Miscible PES blends with P(VP-S) copolymers underwent phase separation on heating caused by LCST-type (lower critical solution temperature-type) phase behavior. Membranes for ultrafiltration process were fabricated from the mixture of PES and P(VP-S) to provide hydrophilic property to the hydrophobic PES membrane, and then their performance was explored. Membranes prepared from the miscible blends of PES and P(VP-S) exhibited better performance in solute rejection and water flux than those prepared from PES or the mixture of PES and PVP, while membranes prepared from immiscible blends of PES and P(VP-S) exhibited worse solute rejection than other membranes. (c) 2005 Elsevier B.V. All rights reserved.