In this study, the solubility, diffusion coefficient, and nucleation behavior of rare-earth metal complexes in a phosphonium-based ionic liquid (IL) were investigated in order to analyze the electrodeposition mechanism of the Dy metal. For the solubility of Dy(TFSA)3 and Nd(TFSA)3 in an IL, the temperature dependence was also evaluated from 340 to 470 K using ultraviolet–visible spectroscopy. The solubility curves obtained for Dy(III) and Nd(III) have similar tendencies and show a relatively good fit with the modified Apelblat equation. A series of thermodynamic parameters (ΔsolG∞, ΔsolH∞, and ΔsolS∞) were also estimated, and the results reveal that the dissolution process of rare-earth metal salts in polar ILs is controlled by enthalpy. For electrochemical analyses, cyclic voltammetric measurements reveal that the reduction process of Dy(III) proceeds in two steps by way of Dy(II) [Dy(III) + e− → Dy(II), Dy(II) + 2e− → Dy(0)], when the IL contains a small amount of water (