Partition coefficients for a series of four dinitrophenylated (DNP) amino acids were experimentally determined in four biphasic systems, composed of hydrophobic ionic liquids (Its) and water. The ILs were 1-alkyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide, [C(n)mim][NTf2] (n = 2, 4, 6, and 8). DNP-amino acids distribute preferentially to the water-rich (top) phase. The experimental partition coefficients decrease as the size of the alkyl side chain of both the DNP-amino acid and the IL increase. The experimental data allowed the calculation of the free energy of transfer of a methylene group (CH2) between the two equilibrium phases, Delta G(CH2). This thermodynamic function is regarded to be a good measure of the relative hydrophobicity of the phases. This property was found to be independent of the size of the alkyl side chain in the ILs. This fact indicates that in this family of ILs the anion contributes much more significantly to the hydrophobicity of the compound than the alkyl chain in the cation.