In this study. effects of Tween 80 (polyoxyethylene sorbitan monooleate) as a variable hydrophilic surfactant additive on morphology and permeability of flat sheet polyethersulfone (PES) membranes prepared from PES/polyethylene glycol (PEG)/n-methyl-2-pyrrolidone (NMP) system via phase inversion induced by immersion precipitation in water coagulation bath were investigated. Cross-sectional morphology of the prepared membranes was studied by scanning electron microscopy (SEM). Permeation performance of the prepared membranes was evaluated in terms of pure water permeability (L-P), water content, porosity, hydraulic permeability and thickness of the prepared membranes. It was found out that little addition of Tween 80 to the casting solution increases water content and porosity of the membrane support layer and enhances pure water permeability through the membranes. (C) 2009 Elsevier B.V. All rights reserved.