[en] The stability constants of the monofluoride complex of Nd(III) and Tm(III) have been determined in mixed solvents of methanol and water at 0.1 mol dm^-3 ionic strength, using a solvent extraction technique. The values for both lanthanides increase as the mole fraction of methanol in the mixed solvent system increases. The variation of the stability constants was accounted for by the Gibbs' free energy of transfer of F^- between water and the mixed solvents. Furthermore, a small effect of cation solvation on the variation of ln β₁ was also interpreted as due to both (1) a low preferential solvation of methanol and (2) a difference between the electrostatic solvation energies of Nd³⁺ and NdF²⁺, and Tm³⁺ and TmF²⁺, in water and the mixed solvent solutions. (orig.)