The manuscript and electronic supplementary information (ESI) of the article with the same title. All ion distribution data can be found in the ESI document. The raw and treated ITC and spectroscopic data (XAS, FTIR, NMR) used in this work can be found in the respectively labelled folders/files. The biphasic transfer of Eu(NO3)3 by trioctylphosphine oxide (TOPO) diluted in a molecular diluent or as a component of a hydrophobic eutectic solvent (HES) was studied by two-phase isothermal titration calorimetry and complemented by XAS, FTIR, and NMR spectroscopy. In HES, the solvent intermolecular interactions introduce an enthalpic penalty, which is overcompensated by a reduction of the entropic cost of Eu(III) phase transfer, resulting in enhanced metal partitioning.