Gadolinium and yttrium show very similar adsorption behaviors on silica. For the both trivalent cations, the adsorption isotherms are insensitive to the ionic strength. The slope of the log-log adsorption isotherms at a constant pH is considerably lower than 1, except for the range of very low equilibrium concentrations. This behavior can be explained without invoking surface heterogeneity considering the presence of a small concentration of anions, An(2-), as an impurity. A hypothetical ternary complex SiOGdAn is very strong and this leads to enhanced uptake at low initial concentrations. When [Gd3+] much greater than [An(2-)], only a small fraction of Gd can be adsorbed as SiOGdAn and the percentage of uptake is low.