The miscibility of poly(ether sulfone), PES, with various hydrophilic copolymers containing 1-vinylpyrrolidone or vinyl alcohol as a repeat unit was explored. Among these blends, 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 lower critical solution temperature type phase behavior. The phase separation temperature of miscible blends first increases gradually with VP content, goes through a broad maximum centered at about 77 wt % VP, and then decreases just prior to the limiting content of VP for miscibility with PES. The calculated interaction energies of PES/P(VP-S) blends indicated that miscibility of these blends stemmed from the intramolecular repulsion between VP and styrene.