The effects of additive, Tween-20, on the morphology and ultra-filtration performance of the PVDF membranes formed by isothermal immersion precipitation from the Tween-20/water/TEP/PVDF system were investigated. High resolution FESEM imaging indicates that the membranes' bulk constituted of interlinked crystalline particles, whose shapes change from more sheaf-like to more stick-like as larger amounts of Tween-20 were initially added into the dope. Introduction of Tween-20 caused formation of nano-pores on the top surfaces of the membranes and micron-sized columnar voids underneath the top skin layer. Some small amounts of Tween-20 may reside in the formed membrane if the nascent membrane has not been carefully washed. Presence of Tween-20 in the membrane was analyzed by water contact angle measurements, Fourier Transform Infrared-Attenuated total reflection spectroscopy (FTIR-ATR), Nuclear Magnetic Resonance (NMR), and X-ray Photoelectron Spectroscopy (XPS). Furthermore, water flux and blue dextran filtration experiments were carried out to illustrate the potential applications of the membranes in fine separation processes. (C) 2014 Elsevier B.V. All rights reserved.