The stability constants, β1, of each monochloride complex of Am(III) have been determined in a mixed system of methanol and water at 1.0 mol·dm−3 ionic strength using a solvent extraction technique. The values of β1 of Am(III) decrease up to about 0.1 mole fraction of methanol (Xs) in the bulk solutions and then increase with increasingXs when 0.1<Xs≤0.4. The distance of Am3+−Cl− in the mixed system was estimated using a Born-type equation. From the estimated distance of Am3+−Cl− (dAm−Cl), it is concluded that AmCl2+ in the aqueous solution is present as a solvent-shared ion-pair. Further, based on the variation of dAm−Cl with increasingXs, the variations of β1 in the system are accounted for by the size-variation of the primary solvation sphere around Am(III) and by an effect due
to the presence of a slight covalency in the solvation of Am(III).