The precipitation of divalent metal cations Cu2+, Zn2+. Ni2+ and Mg2+ with and without A1PO. is measured potentiometrically as a function of temperature and concentration of the divalent metal cations. The shift of the titration curves at a given temperature in the presence of exchanger is related to the metal ions adsorption at the pH valuea below the plateau formation in the titration curves. The order of precipitation of the divalent metal cations is observed to be Cu2+ > Zn2+ > Ni2+ > Mg2+ which is parallel to the values of their solubility products. Law of mass action is applied to distinguish between the ion exchange sorption and precipitation of the metal cations. It is found that the process of the uptake of metal cations changes from adsorption to precipitation with the increase in concentration, pH and temperature of the system.