The fouling of a spiral wound reverse osmosis (RO) membrane after nearly 1 year of service in a brackish water treatment plant was investigated using optical and electron microscopic methods, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Both the top surface and the cross-section of the fouled membrane were analysed to monitor the development of the fouling layer. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. Fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic-Al-P complexes and the particulate matter was mostly aluminium silicates. Subsequently, as the fouling layer reached a thickness of about 5-7microm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).