Study of metal ion adsorption in aqueous solutions using a spin-contamination method has shown that NH4+ can effectively prevent Ca2+ adsorption onto the hydrophilic, oxidized silicon surface. In ammonia-hydrogen peroxide-water solution (APM) the relatively low Ca2+ adsorption on the wet oxidized silicon surface can be explained by its competition for adsorption sites with NH4+ which is the dominant cation in APM. NH4+ prevents Ca2+ adsorption onto the oxidized silicon surface, by an ion-exchange mechanism analogous to that of H+ in acidic solutions. The equilibrium constant governing the adsorption of NH4+ is comparable to, or larger than, that of H+, meaning that NH4+ is at least as effective as H+ in removing metals from the oxidized silicon surface and in keeping metal ions off the surface.