AbstractA macrovoid structure was found in polysulfone membranes formed by the wet‐inversion method using N‐methylpyrrolidinone (NMP) and water as the solvent/nonsolvent pair. The addition of Span series surfactants in the casting solution was found to inhibit the macrovoid formation. Sorbitan monooleate (Span‐80) was more effective in inhibiting macrovoids than was sorbitan monolaurate (Span‐20). The phenomenon of macrovoid suppression became more prominent at higher temperature. The cloud‐point shift might account for the temperature effect. But there was no simple explanation for the effect caused by Span‐80. The cloud‐point position, the demixing rate, and the viscosity of the casting solution were measured. None of them could explain the effect of macrovoid inhibition by the addition of Span‐80. However, the penetration test indicated that the penetration of a nonsolvent into a surfactant‐free casting solution was caused by a convective flow, but that into Span‐80 was through diffusion. It was found that the retardation of nonsolvent penetration was the major cause of macrovoid inhibition. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 166–173, 2002