One of the most important contaminants that ion-exchange resins are exposed to is fouling by organic materials. Especially, anion-exchange resins are more sensitive to fouling by organic materials. The fouling of anion-exchange resins by organic materials is primarily caused by the degradation of products of cation ion exchangers and humic and fulvic acids. Organic fouling causes product water with low quality and few anion exchangers and shortens the service time. Also the need for rinsing water and the use of regeneration chemicals increase. Operating capacity losses occurring due to the fouling of anion-exchange resin by humic acid were quantitatively deter-mined. SAK(254) (Spektraler Absorptions Koeffizient = spectral absorption coefficient), DFZ(436) (DurchsichtsFarbZahl = indexes of transparency), conductivity and sulfate measurements were made to determine capacity losses, which were obtained as 21%, 23%, 25% and 30% after the fouling studies of anion-exchange resin by the amounts of 0.13, 0.25, 0.5 and 1.0 mg/L humic acid, respectively. It was found that even small concentrations of humic acid resulted in a considerable amount of capacity losses in anion-exchange resin.