Chemical adsorption of ions on SiO₂ and Al₂O₃ in aqueous solution is examined. It is revealed that sulfate ions adsorb both on Si and Al oxide surfaces and that the adsorption results in zeta potential decrease. Decrease in zeta potential of Al₂O₃ in NaHCO₃ solutions indicates a chemical adsorption of HCO₃^- ions on the Al oxides. When two different ions, for example, Al(III) and sulfate ions are contained in a SiO₂ dispersed solution, the quantity of each adsorbant on the SiO₂ is almost same as that in the case they are contained separately. Profiles of zeta potential pH for a coexisting system reflect that of a solely existing system.