Surface-complex formation between synthesized amorphous aluminosilicate and Zn ions has been studied by potentiometry. A batch adsorption experiment was performed at 25 +/- 0.1 degrees C while the pH value of the suspension was kept at 6.50 and the solid concentration was kept at 0.1 g/liter, The amount of Zn adsorbed on the aluminosilicate and the amount of H+ released from the solid were determined, The observed molar ratio of released H+ to adsorbed Zn was 2 when the total concentration of Zn in the system was relatively high (1.5 -13 mmol/liter), This was consistent with the charge balance, However, when the total concentration of Zn in the system was relatively low (0-1.5 mmol/liter), the molar ratio of Zn to H+ was found not to be 2, but near 1. This observation suggests the possibility of surface complex formation between amorphous aluminosilicate and a monovalent Zn complex ion, such as Zn(NO3)(+) and Zn(OH)(+)