The physico-chemical properties of the active layer of nanofiltration/reverse osmosis (NF/RO) membranes were characterized by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). NF/RO membranes with active layer materials including polyamide (PA), polyvinyl alcohol derivative (PVA), PA-PVA, and sulfonated-polyethersulfone (SPES) were investigated. SEM and TEM images of membrane cross-sections confirmed the presence of a denser active layer supported by an asymmetric porous polysulfone structure but could only provide a rough estimate of membrane active layer average thickness. RBS provided an accurate tool to determine the elemental composition of NF/RO membrane active and support layers, and the thickness and roughness of the membrane active layer. The oxygen-to-nitrogen molecular ratio obtained for PA membranes was in the range of 1.1-3.0, which is consistent with the reported presence of unreacted carboxyl groups in the active layer surface of this type of membranes. (c) 2006 Elsevier B.V. All rights reserved.