Polyacrylonitrile (PAN) ultrafiltration (UF) membranes were prepared by the pretreatment (gas-liquid interfacial chemical reaction) and the phase inversion process from a casting solution containing dimethylacetamide (DMAC) as a solvent and CaCl2/NH3 . H2O/H2O as a composite additive. Deionized (DI) water was used as a coagulant. The membranes were characterized in terms of the pure water fluxes, protein retention and direct field emission scanning electron microscopy (FESEM) observations. The effects of gas-liquid interfacial chemical reaction on the membrane performance were investigated by changing the pretreatment time, CO2 contacting method (static or flowing) and Partial pressure of CO2. The gas-liquid interfacial chemical reaction had great influence on reducing pore size and increasing porosity of PAN membrane. (C) 2004 Elsevier B.V. All rights reserved.