The suitability of using RBS for the characterization of polymeric composite RO/NF membranes was investigated with the specific goal of determining helium ion fluence thresholds resulting in measurable changes in membrane elemental composition and the thickness/roughness of membrane active layers. Two commercial membranes with polyamide (PA) and sulfonated-polyethersulfone (SPES) active layers on PSf support, and the PSf support without active layer were investigated. Changes in physico-chemical properties of the polymers were assessed by model fitting of the RBS spectra. and analyses with scanning electron microscopy (SEM) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. oxygen was found to be the most unstable element in all three materials investigated (PSf, PA, SPES) thus setting the helium ion fluence thresholds at 3 x 10(14) He+/cm(2) for PSf, 5 x 10(14) He+/cm(2) for PA, and 9 x 10(14) He+/cm(2) for SPES for accurate determination of elemental composition of both the active and support layers. If RBS is used to determine only the thickness and roughness of the membrane active layer, then the helium ion beam fluence could be set below higher thresholds of 2 x 10(15) and 4 x 10(15) He+/cm(2) for membranes with PA and SPES active layers. respectively. (c) 2007 Elsevier B.V. All rights reserved.