A new hydrophobic eutectic solvent (HES) di(2,4,4-trimethylpentyl)phosphinic acid/menthol was first proposed. HES was characterized by NMR and IR spectroscopy (confirmation of intermolecular interactions - hydrogen bonds) and its thermal stability was studied by differential scanning calorimetry. Its basic physical properties (density, shear and dynamic viscosity, refractive index, etc.) as a function of temperature in the range 15-60°C have been studied. The possibility of HES application as an extractant for the selective extraction of transition metal ions in a heterogeneous liquid-liquid system has been investigated.