The stability constants (β1) of iodide ion-pairs of trivalent f-block element ions (lanthanoids Ce, Eu, Gd, Tb and Tm, and actinoid Am) were determined
in the vicinity of pH 2.5 of mixed methanol/water solvent solutions of an ionic strength of 1.00 mol·dm−3 at 298±1 K. The values were less than 2. From the variation in distance between Eu3+ and I−, which was calculated using a Born-type equation for Gibbs’ free energy derived from β1(Eu), the Eu3+-I− interaction was shown to be solvent-shared ion-pair formation when the mole fraction of methanol (XMeOH)≤0.40. In contrast, it was suggested that the interaction of Am3+-I− changed from solvent-separated ion-pair to solvent-shared ion-pair with increasing XMeOH when XMeOH≤0.10, but remained as solvent-shared ion-pair in the range 0.16≤XMeOH≤0.40. Furthermore, β2(Am) was measured in the range 0.31≤XMeOH≤0.40. It was also shown that the β1 values of lanthanoids at XMeOH = 0.40, except for that of Gd(III), decreased with increasing atomic number.