Polyethersulfone microfiltration (MF) membranes can be produced from polyethersulfone/solvent/nonsolvent systems by the immersion phase inversion process in a water bath. When the casting solution composition is adjusted close to phase separation, a fast gelation in water occurs which hinders further polymer enrichment at the interface between polymer solution and water; an open-cell MF membrane structure is thus retained. Addition of a certain amount of a water-soluble polymer, polyvinylpyrrolidone, to the casting solution will further change the demixing rate and gelation rate of the casting solution in water. Such an addition improves the open-cell structure of the membranes. Besides dope composition, the performance of these MF membranes is also dependent on the casting conditions.