Ion exchange technologies have commonly been applied, i.e., in demineralization, softening and dealkalization as the standard water treatment processes. Certain changes appear in the ion exchange material during its lifetime, as it cycles from its regenerated form to an exhausted form. These changes are frequently caused by fouling of ion exchange resins. When water, containing objectionable substances, comes into contact with resin, impurities clog the pores and coat the resin surface. In such cases, ion exchange mechanism may become inhibited producing less quantities of water with poor quality and also decreasing the resin's life span. Almost all water supplies contain iron. Iron fouling of ion exchange demineralizer results in an increase in the effluent level of iron, a reduction in operating capacity and possibly also in back-washing efficiency under identical operating conditions. Because of pore clogging and degradation of functional groups, particularly anion resins must be prevented from the iron fouling. In this study, changes on the operating capacity of ion exchange demineralization in the presence of different iron ion concentrations (0.2, 0.5, 2.0, 4.0 and 8.0 mg/L) were investigated. It was found that fouling of demineralization system with iron, even at low concentrations (0.2 mg/L), led to a loss in operating capacity (6%). When iron concentration was increased to 0.5, 2.0 and 4.0 mg/L, the loss of operating capacity was found to be about 12-13 %, and increasing to 16% when iron concentration was 8.0 mg/L.