Ferrous iron has been considered very useful for removal of chlorite, a by-product of chlorine dioxide. However, a serious problem can occur associated with high residual iron concentrations in water after Fe(II) treatment, especially at weak acidic condition (pH 6), having a higher capacity of chlorite reduction. Simultaneous removal of chlorite, dissolved iron, and humic acids was achieved with a Fe(II) –Al(III) (1:5) coagulant mixture. Moreover, a significant reduction of chlorine by-products formed after subsequent post chlorination was observed. The by-products of chlorine dioxide and chlorine could be dramatically reduced by the simple addition of the coagulant mixture without the additional problem of dissolved iron.