Tunisia is a semi and country; the shortage of good water quality in particular at the south regions forced the government to desalt brackish water via membrane technology such as RO to supply these regions with potable water. Station of Djerba. Isle established in 2000 by SONEDE has a nominal capacity of 15 - 10(3)m(3) per day and recovery rate of 75%; it is fed from an artesian well with the water salinity of 5500 ppm. Three RO units compose station; each one has configured in two stages a 2:1 array with six RO polyamide elements per pressure vessel. Membranes have been cleaned or replaced, during four operating years, as much time. In the aim to determine the reason of membrane fouling, this work occurs to analyse fouling layers. The membrane autopsy has achieved by different analytical methods, such as COT measurement, SEM (scanning electron microscopy), IR analysis, and diffraction by X-ray. Obtained results show that the deposit composed mostly of organic matter, silica, iron and CaCO3. The debris accumulate on upstream of pressure vessel is composed mainly of silica, metallic matter (iron and chromium) and aluminium phosphate. The presence of TOC indicates the presence of microorganism. The metallic products come from stainless steel fittings and internal coating film of cartridges filter that are damaged by the silica particle probably rejected by sand filters. The scale of CaCO3 deposition is owing to the bad anti-scaling organophosphonate-based, which can form aluminium phosphate by reaction with aluminium composing cartridges filter internal coating.