The enthalpies of mixing of aqueous imidazole solution and six kinds of amino acid solution (glycine, L-alanine, L-serine, L-valine, L-proline, L-threonine) have been determined at 298.15 K using a 2277 flow microcalorimetry. The results have been analyzed in terms of the McMillan-Mayer model to obtain the heterotactic enthalpic interaction coefficients. The pairwise interactions between amino acids and imidazole have been studied by a group additivity approach of savage and wood (abbreviated as SWAG). The results show that the zwitteriomic groups and different side-groups of the amino acids make their different contribution to enthalpic pair interactions. (C) 2003 Elsevier B.V. All rights reserved.