Partitioning of 153 di- to hexapeptides in an aqueous dextran-PEG two-phase system containing 0.15 m NaCl in 0.01 m sodium phosphate buffer, pH 7.4 was examined. The relative hydrophobicity of the peptides was estimated and expressed in equivalent numbers of methylene units. Analysis of the data shows that the additivity principle applies for the relative hydrophobicity of up to hexapeptides. The relative hydrophobicities of Trp, Glu, and Asp residues in heterooligopeptides are noticeably different from those in corresponding homooligopeptides. The relative hydrophobicity of peptides can be calculated and used as a structural descriptor in quantitative structure-activity relationship analysis. The peptide bitterness threshold is shown to be quantitatively related to the peptide structure described as a combination of the relative hydrophobicity and lipophilicity (logD) of peptides.