To find the laws governing the extraction of cations from aqueous solutions into hydrophobic ionic liquids (ILs), we investigated the partition of 1-ethylpyridinium monocation and paraquat (1,1'-dimethy1-4,4'-bipyridinium) dication in various IL/water biphasic systems. Ten different ILs of 1-butyl-3-methylimidazolium-based or bis-(trifluoromethanesulfonyl)amide-based salts were used. The distribution ratio of the target cations (Tn+) was dependent on the initial concentration in the aqueous phase and also very sensitive to the kind of IL. The behavior was quantitatively explained on the basis of a model in which the extraction goes through both the ion exchange and ion pair transfer processes, while keeping the product of the aqueous concentrations of the IL constituent ions a constant value (solubility product, K-sp). The distribution ratio of Tn+ is expressed as a function of the difference between the initial and equilibrium concentrations of Tn+ in the aqueous phase (Li [Tn+](w)), the aqueous solubility of IL (K-sp(1/2)), and the cation valence n. The distribution ratio is a nearly constant value (D-0) when Delta [Tn+](w)<< K-sp(1/2)/n and decreases inversely proportional to the nth power of Delta [Tn+](w) when Delta[Tn+](w) >> K-sp(1/2)/n. The log D-0 versus log K-sp(1/2) plot gives a linear relationship with a slope of +n for the ILs with the same anion but different cations and that with a slope of nearly n for the ILs with the same cation but different anions. This means that the extractability dependence on the kinds of IL constituent ions is greater for the divalent cation than for the monovalent one.