Surface complex formation between synthetic amorphous aluminosilicate and Zn2+ ion in 0.1 N NaNO3 was studied at 25 +/- 0.1 degrees C by potentiometry. The concentration of Zn2+ in the suspension was varied, while the concentration of the aluminosilicate was kept constant at 1 g air-dried aluminosilicate in 1 1. The pH value of the suspension was kept constant at 6.50 by 0.01 N NaOH with an automatic titration system to neutralize H+ released from the solid phase. The amount of Zn(II) retained on the solid phase was calculated from the difference between the total Zn2+ concentration in the suspension and the equilibrated concentration of Zn2+ in the liquid phase. The molar ratio of Zn(II) retained on the solid surface to H+ released from the solid phase is 1:2, suggesting ion exchange of two surface HS and one aqueous Zn2+ ion with conservation of charge balance. The surface complex formation between synthetic amorphous aluminosilicate and Zn2+ ion in aqueous solution can be described as 2{Al, Si-OH} + Zn2+(aq) = {(Al, Si-O)(2)Zn} + 2H(+)(aq), where { } represents the chemical species on the solid surfaced.