Cu2+ ion adsorption and potentiometric titration studies were carried out on AIPO(4) as a function of pH, temperature, and activation. Two different samples thermally treated at 105 and 400 degrees C were employed for the sorption and potentiometric titration studies. The dissociation constant values (pK(a)) showed an increase with thermal activation, indicating a decrease in the surface acidity of AlPO4. Cu2+ ion adsorption was found to increase with pH, temperature, and thermal activation of the solid. The Freundlich equation was found to be applicable to the sorption data.