Recent epidemiological studies on arsenic carcinogenicity have inspired increased concern over the levels of arsenic in drinking water and a re-evaluation of the current drinking water standard. Enhanced coagulation is one of the treatment technologies that may be applied for arsenic removal. The efficiency of arsenic removal from source waters by addition of coagulants, such as alum or ferric chloride, depends, in part, on the extent of arsenic adsorption to freshly-precipitated amorphous metal hydroxides. The surface complexation model for adsorption provides some insight into the effects of arsenic concentration and oxidation state, coagulant dose, pH, and source water composition on arsenic removal by coagulation treatment.