Forward osmosis (FO) has a potential role in desalination and water reclamation. However, the efficiency of FO processes is significantly limited by internal concentration polarization (ICP), which is difficult to detect by conventional characterization methods. In this work, we aimed to apply electrochemical impedance spectroscopy (EIS) to FO systems, and to develop a methodology for interpreting the interplay between the membrane structure and the polarization phenomena. Three commercial FO membranes with different substrate structures were characterized by an EIS system that was modified to accommodate FO processes, and the impedance spectra were analyzed based on their equivalent circuits. In the static tests (without osmosis), the limit behaviors of the impedance spectra were exploited to resolve the structural information of the FO membranes from the electrolyte background, which was in turn compared with the characterization results via scanning electron microscope (SEM). Associated with the different membrane orientations, the dynamic tests (with osmosis) were implemented to verify the ability of the EIS system to detect the variation of the developed concentration profiles within the membrane structure. The characterization results indicate that EIS is promising to be a handy tool to investigate the polarization phenomena during the FO processes. (C) 2012 Elsevier B.V. All rights reserved.