The binding contants, adsorption free energies, absolute adsorbate number densities, and interfacial charge densities of Al(III), Sc(III), Y(III), La(III), and Gd(III) interacting with fused silica/water interfaces held at pH 4 were determined using second harmonic generation and the Eisenthal chi((3)) technique. By examining the relationship between the measured adsorption free energies and the electric double layer interfacial potential at multiple electrolyte concentrations, we elucidate the charge state and possible binding pathways for each ion at the fused silica surface. Al(III) and Sc(III) ions are found to bind to the fused silica surface as fully hydrated trivalent species in a bidentate geometry. In contrast, the Y(III), La(III), and Gd(III) ions are each shown to adsorb to the silica surface in a decreased charge state, but the extent and mode of binding varies with each ion. By quantifying the exponential sensitivity of the surface coverage of the adsorbed ions to their charge state directly at the fused silica/water interface, we provide benchmarks for theory calculations describing the interactions of metal ions with oxide interfaces in geochemistry and hope to improve the prediction of trivalent metal ion transport through groundwater environments.