A study of competitive adsorption of Ca2+ and Zn(II) ions at the monodispersed SiO2/electrolyte solution interface is presented. Influence of ionic strength, pH, and presence of other ions on adsorption of Ca2+ and Zn(II) in the mentioned system are investigated. zeta potential, surface charge density, adsorption density, pH(50%), and DeltapH(10-90%) parameters for different concentrations of carrying electrolyte and adsorbed ions are also presented. A high concentration of zinc ions shifts the adsorption edge of Ca2+ ions adsorbed from solutions with a low initial concentration at the SiO2/NaClO4 solution interface to the higher pH values. This effect disappears with a concentration increase of calcium ions. The presence of Ca2+ ions in the system slightly affects the adsorption of zinc ions on SiO2, shifting the adsorption edge toward lower pH values and thereby increasing the adsorption slope. (C) 2003 Elsevier Inc. All rights reserved.