An approach to optimize the membrane performance by controlling the acidity or basicity of the diamine solution containing bifunctional piperazine, trifunctional 3,5-diaminobenzoic acid and acidic/basic additive is introduced. It was found that with the addition of sulphuric acid (H2SO4), pore radius becomes bigger and the skin layer becomes thinner. On the other hand, the membrane thickness was found to increase with the addition of triethylamine (TEA) whereas the pore radius was not affected by the addition of TEA. The membrane flux increased when the membrane was prepared under acidic condition, however, the mechanical strength of the membrane pores was poor and subjected to severe compaction problem. CuSO4 rejection under different pore radius was simulated using Donnan steric pore model (DSPM). The simulation result shows that the optimum pore radius to separate CuSO4 should be as close as possible to the size of hydrated Cu2+ ion which is around 0.53 nm. The experimental rejections for membrane with different pore radius were compared to the simulated result. It was found that the trend of rejection agreed well with the simulated data which indicated that optimized CuSO4 removal could be achieved using membrane prepared with the addition of 0.5% sulphuric acid. (C) 2005 Elsevier B.V. All rights reserved.