Relatively hydrophobic polyamide (PA) membranes exhibit lower flux rates during aqueous separations but can accommodate different solvents for various separation applications. Since most of the information on commercial reverse osmosis (RO) membranes is proprietary, further investigation on their structure would be beneficial for selection of the proper membranes especially for processes involving different solvents. Four commercially available RO membranes, AK, AG, SE and SG, were characterized using contact angle, XPS, ATR-FTIR, FE-SEM and AFM measurements. All membranes were found to be relatively hydrophobic with contact angles greater than 60 degrees. The findings suggest that hydrophobicity of AK and AG were due to close intermolecular chain distances providing intermolecular hydrogen bonds while hindering water-polymer interactions. In the case of SE and SG membranes, an additional polymer, PVA, was probably involved during polymerization forming a polyesteramide structure resulting in reduced number of sites for water to interact with. Understanding the chemical structure associated with the hydrophobic network of the PA membranes is necessary to achieve the targeted separation without sacrificing membrane efficiency. (C) 2011 Elsevier B.V. All rights reserved.