In this work, the role played by surfactants in the formation of PMMA membranes was investigated. For a wet inversion process with PMMA as polymer, acetone as solvent, and water as coagulant, delayed demixing was observed and the resulting membrane possessed a sponge-like structure. Adding 1.8 vol% of Tween 80 in the casting solution (PMMA + acetone) could shift the PMMA/acetone/water system from delayed demixing to instantaneous demixing and could induce macrovoids with small pin-holes on the membrane surface. It was found that the affinity of surfactant for coagulant is crucial for the enhancement of the formation of macrovoids. When water was the coagulant, adding the surfactants with high water solubility in PMMA/acetone solution can enhance the formation of macrovoids while adding the surfactants with low water solubility has no effect. On the other hand, for a lipophilic coagulant such as n-hexane, lipophilic surfactants were more effective in changing membrane structure than hydrophillic surfactants. In addition, a mechanism describing the role of surfactants in the formation of macrovoids is proposed : the addition of appropriate surfactants can enhance the affinity between solvent and coagulant, resulting in a shift from delayed demixing to instantaneous demixing, and macrovoids can then be induced.