Chemical adsorption of Al and Rh ions on SiO₂ in aqueous solutions are examined. It is revealed that complex cations, such as [Al(H₂O)₆]³⁺, [Al(OH)(H₂O)₅]²⁺ and [Al(OH)₂(H₂O)₄]⁺, are adsorbed on SiO₂ in the pH range 4 to 9 and that the adsorption results in the shifts of values in zeta-potential up to ca. +60 mV. On the other hand, Rh ions, which are inert in the pH range, are not adsorbed on SiO₂ and no substantial differences are observed between values of zeta-potentials in Rh solutions and those in metal-free solutions. In the pH range 10 to 12, however, it is observed that both Al and Rh ions are adsorptive on SiO₂. In All Rh containing solutions, the values of zeta-potential are between -50 to -60 mV, which are about 10 mV smaller than those in metal-free solutions. Therefore, it is supposed that complex anions, such as [Al(OH)₄(H₂O)₂]- and [Rh(OH)₄(H₂O)₂]^-, are adsorbed in the alkaline solutions. As for the mechanism, we propose that the adsorption proceeds through ligand-exchange reactions of hydrated Al/Rh ions.