The dissolution rate of amorphous silica is enhanced by sulfate ions. The zeta potential for silica particles in Na2SO4 solution was lower than that in Nacl solution with the same ionic strength. These facts indicate that the specific adsorption of sulfate ions occurred by overcoming repulsion between negative charges of the SO42- ion and Si-O- on the Surface of silica. The dissolution rate of amorphous silica may be accelerated by the specific adsorption of SO42- ions ions because of a decrease in the strength of the Si-O-Si bond in amorphous silica due to donation of electron density from the adsorbed SO42- ions. (C) 2008 Elsevier Inc. All rights reserved.