Zeta potentials of colloidal particles were used to study the influence of anions on the formation of ferric oxide hydroxides [Fe(O)OH]. This anion effect, complexation with ferric ions and adsorption on the resulting particle surface, correlated well with the determined zeta potentials. The greater anion effect in ferric ion solution decreased the measured zeta potential of colloidal particles. This effect increased with increasing Size, charge and complexation of anions. The results indicated that the composition and morphology of Fe(O)OH precipitates were affected by the presence of anions in solution.