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The retention of 19 solutes (Ala, Gly, Lys, Phe and their homopeptides) has been determined on impregnated alumina layers using various mixtures of water-methanol as mobile phases. Lipophilicity and specific hydrophobic surface area of solutes were estimated from linear correlations between the retention of solutes and the concentration of methanol in the mobile phase. The relationship between the hydrophobic retention parameters and physicochemical parameters has been elucidated by the principal component (PC) analysis followed by nonlinear mapping carried out on both the original and absolute values of PC loadings and variables. Ala, Gly, Lys and their homopeptides showed anomalous behavior, the retention increasing with increasing the concentration of methanol in the mobile phase. The use of absolute values in the calculations enhanced the reliability of the mapping technique. It was found that other than hydrophobic forces are involved in the solute stationary phase interaction (mixed retention mechanism).