[en] Ionic hydration and electrostriction are very important phenomena in various fields such as medical science, biochemistry, and chemical engineering. The present study is the first to focus on these parameters at infinite dilution. Limiting partial molar volumes (V₂₃^∞) in a solvent (1) − [solute (2) + solute (3)] system were measured at several solute compositions x₃ [= n₃/(n₂ + n₃)] at 30.00 ± 0.05 °C. When the mixed solutes (2 and 3) only slightly ionized, V₂₃^∞ were observed on the additivity line. This result was attributed to the fact that only solvent (1) molecules can exist around an isolated solute (2) molecule or an isolated solute (3) molecule at infinite dilution. However, V₂₃^∞ in water − [pyridine + fatty acid (acetic acid, propionic acid, or butyric acid)] systems negatively deviate from the additivity line. By measuring electrical conductivity (κ) and mixing enthalpy (ΔH_mix), the negative deviations of V₂₃^∞ from the additivity line were attributed to the ionic hydration of pyridine and the fatty acid. As a result, by measuring V₂₃^∞ at several solute compositions, the electrostriction in water − [pyridine + fatty acid] systems was observed. Conversely, V₃^∞ in all 14 [solvent (1) + solvent (2)] − solute (3) systems deviated in complicated ways from the additivity line. Unfortunately, we could not clarify the reason behind this deviation because numerous factors had to be considered. Thus, it was confirmed that the behavior of $V_3^{\mathrm{\infty <!--\; ???\; -->}}$ is much more complicated than that of V₂₃^∞. The solvents used to study V₂₃^∞ and $V_3^{\mathrm{\infty <!--\; ???\; -->}}$ are as follows: water, pyridine, acetic acid, propionic acid, butyric acid, benzene, acetone, chloroform, methanol, ethanol, DMF, carbon tetrachloride, isobutylamine, isobutyl alcohol, n-heptane, and n-octane.