The enthalpies of solution of L-a-aspartic acid, L-a-glutamic acid, L-a-arginine, L-a-lysine, and L-a-histidine have been measured in aqueous ethanol solutions at 298.15 K. From the obtained experimental results, the standard enthalpies of solution of amino acids in waterethanol solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan-Mayer's formalism. These values were interpreted in the terms of the ionic or no polar effect of the side chains of L-a-amino acids on their interactions with a molecule of ethanol in water. Keywords L-a-Amino acids Á Ethanol Á Aqueous solution Á Enthalpies of solution Á Enthalpic pair interaction coefficients List of symbols h A,EtOH The heterogeneous enthalpic pair interaction coefficient between zwit terions of amino acids and ethanol molecule h A,EtOH,EtOH The enthalpic triplet interaction coef ficient D sol H 1 m ðWÞ Standard enthalpies of solution of L-a-amino acids in water (W) D sol H 1 m ðW þ EtOHÞ Standard solution enthalpies of the La-amino acids (A) in ethanol (EtOH) and water (W) mixtures m EtOH Molal concentration of ethanol in water (mol kg-1