The phase-inversion process was used to prepare integrally skinned asymmetric polysulfone (PSf) membranes with different pore sizes. Membranes were prepared from a casting solution of PSf; N-methyl-2-pyrrolidone (NMP) as solvent; and 1,4-dioxane, diethylene glycol dimethyl ether (DGDE), acetone, and gamma-butyrolactone (GBL) as additives by immersing them in water as a coagulant. The effect of the additives on membrane performance and structure was investigated. The low miscibility of 1,4-dioxane, DGDE, and acetone with the coagulant resulted in reduced membrane pore size. However, by using GBL as additive pore size of the membrane was slightly increased because of its higher miscibility with the coagulant than NMP. Changing the amount of additives in the casting solution could control the molecular-weight cutoff values of asymmetric membranes. (C) 2003 Wiley Periodicals, Inc.