The interaction of the Zn2+ and Ni2+ ions at the iron (III) hydroxiede/solution interface was studied as a function of the constant initial pH and over the metal ion concentration range 1-6 ppm. For each mole of heavy metal ion adsorbed, approximately two moles of H+ ions were displaced from the interface. These results are used for expressing the adsorption data as a function of the solution concentration of both H+ and metal ions. The results obtained permit the interfacial reaction to be described by the equation: 2SH + M2+ = S2M + 2H+ Were S represents a surface site.