Eects of silicate, sulfate, and carbonate on the removal of arsenite [As(III)] and arsenate[As(V)] by coprecipitation with ferric chloride were studied. Silicate signi®cantly decreased As(III)removal when Si concentration was higher than 1 mg/l and the pH was greater than 5. The removal of As(V) was decreased moderately by silicate in 0.04 M KNO3 solution. Addition of Ca2+ and Mg2+ to the KNO3 solution reduced the adverse aect of silicate on As(V) removal.