Enthalpies of solution of water in pyridine and its methyl derivatives were investigated at T = 298.15 K with a titration calorimeter. The effects of solution are exothermic and increase in the following order: pyridine < 3-methylpyridine < 4-methylpyridine < 2-methylpyridine < 2,6-dimethylpyridine < 2,4,6-trimethylpyridine. As expected, the negative enthalpies of solution are fairly large, especially those for pyridine derivatives with a methyl group in the ortho position, due to the formation of hydrogen bonds between water and amine. Thus, the energy of the N (...) H-O bond, strengthened by the ortho methyl group, overcomes the hindering effect of that group. The experimental results were correlated with the association energies of the 1:1 (water + amine) complexes and with the net charges on the nitrogen atom reported in the literature. Although semi-quantitative agreement between the correlated quantities is fairly good, a simple model based on the hydrogen bond interactions was found to be too simple to account for the measured heat effects. (C) 2003 Elsevier Science Ltd. All rights reserved.