The miscibility of polysulfone (PSf) with various hydrophilic copolymers was explored. Among these blends, PSf gave homogeneous mixtures with poly(1-vinylpyrrolidone-co-styrene) [P(VP-S)] copolymers when these copolymers contained 68-88 wt % 1-vinylpyrrolidone (VP). Miscible PSf blends with P(VP-S) copolymers underwent phase separation on heating caused by lower critical solution temperature (LCST)-type phase behavior. The phase behavior depended on the copolymer composition. Changes in the VP content of P(V-P-S) copolymers from 65 to 68 wt % shifted the phase behavior from immiscibility to miscibility and the LCST behavior. The phase-separation temperatures of the miscible blends first increased gradually with the VIP content, then went through a broad maximum centered at about 80 wt % V-P, and finally decreased just before the limiting content of VP for miscibility with PSf. The interaction energies of binary pairs involved in PSf/P(VP-S) blends were evaluated from the phase-separation temperatures of PSf/P(VP-S) blends with lattice-fluid theory combined with a binary interaction model. The decrease in the contact angle between water and the membrane surface with increasing VP content in P(VP-S) copolymers indicated that the hydrophobic properties of PSf could be improved via blending with hydrophilic P(VP-S) copolymers. (C) 2003 Wiley Periodicals, Inc.