Partitioning experiments for a series of dinitrophenylated (DNP) amino-acids have been conducted in three (ionic liquid + sodium phosphate buffer) biphasic systems at 23 degrees C. The ionic liquids used were 1-hexyl-3-methylimidazolium tetrafluoroborate ([C(6)mim][BF4]), 1-octyl-3-methylimidazolium tetrafluoroborate ([C(8)mim][BF4]), and 1-decyl-3-methylimidazolium tetrafluoroborate ([C(10)mim][13174]). The pH of the initial aqueous solution has been fixed at 7.4, using a sodium phosphate buffer. DNP-amino acids distribute preferentially to the ionic liquid phase, and this partition increases with the length of the alkyl chain of the amino acid. The Gibbs free energy of transfer for a methylene group is calculated from the partition coefficients of the solutes. This provides a measurement for the relative hydrophobicity of the conjugated phases, and the obtained values are compared to those of other biphasic systems used in biotechnology (e.g., polymer-polymer and polymer-salt aqueous two-phase systems).