Long time uptake arsenic will cause cancers and blackfoot disease. There are still several million people suffering from drinking arsenic contaminated water. This work studied the performance of coupled chitosan/UF for arsenic removal and the influence of dissolved organic matter on arsenic removal with UF operation. Humic acid representing dissolved organic matter was fractionated into five groups of molecular sizes by gel filtration chromatography. Arsenic rejection by UF under the experimental condition is only 10%. In the presence of humic compounds, the arsenic removal of 22% is obtained. DOM with apparent molecular weight > 35,000 Da is the principle component responsible for chelating arsenic and thereafter being rejected by UF membrane. The combined interactions of humic compounds, chitosan, and arsenic enable a 65% arsenic rejection by UF. The results presented here enable our understanding of the complicated mechanisms involved in chitosan/UF/DOM/arsenic system. (c) 2007 Elsevier B.V. All rights reserved.