The assessment of long-term fouling in reverse osmosis (RO) membranes was investigated through extensive membrane autopsy using different analytical techniques. The RO membranes were taken from a seawater desalination plant after 4 years of operation. Chemical and structural characterization was performed using different analytical methods, including visual observation, optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectroscopy (AAS) and ion chromatography (IC). The fouling layer consisted of particulate matter embedded in an apparently amorphous matrix, which was unevenly distributed over the membrane surface with the greatest accumulation in the valley areas. Inorganic and organic foulants were identified in the RO membranes. Inorganic foulants mainly consisted of hydrogen aluminosilicates and halite. The main organics substances identified were proteins, polysaccharides and humic compounds and were attributed to biofouling. Chemical analysis revealed that Si, Al, Cl, Ca, Na, Mg and K were the predominant elements contributing to membrane fouling. The reasons for the deposition of foulants on the RO membranes are the increase of membrane selectivity due to biofouling, the large size of cartridge filters and the high operating pressure. (C) 2012 Elsevier B.V. All rights reserved.