In the previous research, ultratiltration membranes exhibiting excellent performance were produced from the miscible blends of polysulfone (PSf) and poly(1-vinylpyrrolidone-co-acrylonitrile) (P(VP-AN)) copolymers. PSf formed miscible blend with P(VP-AN) copolymers containing AN from 2 to 16 wt%, while other P(VP-AN) copolymers were not miscible with PSf. In this study, microfiltration membranes are prepared from immiscible blends of PSf and P(VP-AN) copolymers via traditional phase inversion process. Even though compatibility of PSf/P(VP-AN) blends depends on the AN content of copolymer, the average pore size existing on the membrane surface was about 0.25 mu m regardless of copolymer composition. These micropores stemmed from solvent-nonsolvent exchange channel formed at around the interface between PSf and P(VP-AN) when blends contain less than about 30 wt% of copolymer. Macropores (2 mu m) originated from the dispersed particles (or copolymer rich phase) was observed on the membrane surface along with micropores when the blend contained more than 40 wt% copolymers. (c) 2005 Elsevier B.V. All rights reserved.