The make-up water for the boilers at the Tabriz power plant in Iran is treated by a reverse osmosis (RO) process. The membrane modules used are DuPont B-9 and DuPont B-10. The hollow fine-fibres in the modules are asymmetric polyaromatic amide (Aramid) membranes. RO has been used as part of the water treatment process for more than 30 years. Although the membrane modules are changed regularly, membrane fouling remains a serious problem. To be able to minimize fouling, an investigation into the fouling mechanism is vital. In the following study, the types of foulants on the membrane surface and their origin were identified. The distributions of foulants in various parts of the module were investigated. In the internal part of the module, magnetite (Fe3O4) was the main foulant, probably reflecting from the corrosion the system's components. The main foulants in the external parts of the module were calcium compounds (CaSO4 & CaCO3). Magnesium carbonate (MgCO3) was evenly distributed throughout the module. The main source of of the salts is likely to be the feed water.