The liquid-liquid demixing process of cellulose acetate (CA) in two nonvolatile solvents N-methyl-2-pyrrolidone (NMP) and gamma-butyrolacetone (GBL) coagulating in water was investigated. Due to the relatively "poor" solvency of GBL for cellulose acetate and the higher interaction parameter between GBL and water, the CA/GBL/H2O system exhibits a delayed liquid-liquid demixing, while the CA/NMP/H2O has an instantaneous one. The solvent quality and the interaction parameter between nonsolvent and solvent determine the different liquid-liquid demixing behavior and the distinct membrane structures. By adjusting, the GBL content in the solvent mixture, the effect of solvent on the membrane structure and performance was systematically studied. The membrane structure was tunable by changing the ratio of NMP/GBL. A critical ratio of NMP/GBL appears to exist that denotes the transition of membrane structure from macrovoids to sponge-like. (c) 2006 Elsevier B.V. All rights reserved.