The stability constants (b1) of the monofluoro complex of Cm(III) have been determined in mixed solvents of methanol and water using the solvent extraction technique. The values of lnb1 increase as the molar fraction of methanol (Xs) in the mixed solvent increases. The variation in the stability constants mainly depends on the solvation of F- and slightly depends on both (1) the solvation of cations in connection with the complexation of CmF2+ and (2) the electrostatic attraction of Cm3+-F-. The variation in lnb1 for Cm(III) due to the effect of both (1) and (2) is similar to that for Sm(III). By variation of lnb1 the coordination number in the primary hydration sphere (CN) of Cm(III) decreased from a value between CN = 9 and CN = 8 to CN = 8, at about Xs = 0.02. The Xs value of the inflection point of the CN for Cm is slightly lower than Xs = 0.06 for Sm(III) and Xs = 0.03 for Eu(III), previously obtained.