A detailed thermodynamic examination of the selective extraction of Am3+ from Eu3+ by two dithiophosphinic acids was performed using DFT. By examination of two extractants with two metal ions, the most uncertain terms of these calculations were eliminated, resulting in free energies (Delta Delta Delta G(ext)) that are directly related to the selectivity data. The calculated relative selectivities agree well with experimental data, indicating that the extraction factor is primarily due to the binding free energy of the ligands to the metals and is not dependent on side reactions or complicated solvent effects.