Selectivity coefficients KHMfor trivalent metal ions / hydrogen ions (M3+/H+) exchanges on titanium antimonate cation exchanger (TiSbA) have been determined for group 13 metals (Al3+, Ga3+, and In3+) and rare earth metals (La3+, Nd3+, Eu3+, Yb3+, and Y3+) ions by batch equilibration at 30, 45, and 60°c in nitric acid media. Kielland plots (logKHMvs. the fractional exchange X¯M) for these exchange systems have been analyzed and found to show a break point at X¯M=0.005-0.01. This behavior can be interpreted assuming a two-site model. A site available at infinitesimal X¯Mvalue has a large steric effect. The selectivity in this site increased in the order: Al3+< Ga3+< In3+for group 13 metal ions and Y3+< Yb3+< Eu3+< Nd3+< La3+for rare earth metal ions in the X¯M<0.02 region at 30°C. The amounts of Ga3+exchanged by TiSbA increased with a rise in temperature. The selectivity of group 13 metal ions changed to: AI3+< In3+< Ga3+at60°C. On the other hand, the selectivity order for rare earth metal ions was independent of temperature. The hypothetical thermodynamic data (ΔGoideal, ΔHoidealand ΔSoideal) at 25°C were evaluated.