The coagulation of colloidal MnO 2 by several divalent cations, both in the absence and presence of a complexing ligand (l-histidine), has been studied. The efficiency as coagulating agents increases in the order Mg 2 + <Ni 2 + <Zn 2 + <Cu 2 + <Co 2 + <Mn 2 + . For a given cation, the coagulating efficiency increases with increasing pH and with increasing temperature, and decreases with increasing concentration of l-histidine. Apparent values for the equilibrium constants of complexation of divalent cations by l-histidine have been obtained. The apparent equilibrium constant of formation of the complex increases in the order Mg 2 + <Mn 2 + <Zn 2 + <Cu 2 + <Ni 2 + . The value corresponding to Co 2 + could not be determined because l-histidine acted as a catalyst for the reduction of colloidal MnO 2 by that cation.