During the use of ion exchange resins in water treatment processes (loading, backwashing and regeneration), some unwanted capacity losses may occur. The reason for this is generally the fouling of ion exchanger beds. Aluminium ions which may exist in water can reduce ion exchanger capacities. The fouling, of ion exchangers with aluminium occurs in ion exchange systems which are applied after the coagulation with alum. There is still a need for quantitative results on such fouling and capacity losses that result from coagulation process. In this study, the fouling of strongly acidic and strongly basic ion exchange resins with aluminium was examined. The capacity losses of ion exchangers were determined quantitatively after the fouling of ion exchange resins with various aluminium concentrations (0.1, 0.5, 1.0, 2.0, 4.0, 8.0 and 10.0 mg/L). The capacity losses found in strongly acidic cation exchanger were found to be 10%, 12% and 19% for the aluminium concentrations of 0.1, 0.5 and 1.0 mg/L, respectively, but 22% for 2.0 and 4.0 mg AI(III)/L and 29% for 8.0 and 10.0 mg AI(E[I)/L. The capacity losses found in strongly basic anion exchanger were 29%, when fouled with 0.1 and 0.5 mg AI(Ell)/L, 30% for 1.0 and 2.0 mg AI(III)/L, and 32% when aluminiu amount was increased to 4.0, 8.0 and 10.0 mg/L.