Excess enthalpies for the mixtures of methylpyridine isomers with light and heavy water were measured at T = 298.15 K within the whole concentration range. The excess partial enthalpies were calculated. The negative values of the excess enthalpies for {xC(6)H(7)N + (1 - x)H2O} are slightly larger than those for {xC(6)H(7)N + (1 - x)D2O}. It was suggested that the differences between H2O and D2O, reflected in the first enthalpies of solution of C6H7N, could be due to a hydrophobic effect in dilute aqueous solutions in spite of the polarity of the methylpyridine molecules. The enthalpies of the solution of 2- and 3-methylpyridine in D2O, significantly smaller than those in H2O, do not compensate the large entropy effect due to the rearrangement of the solvent structure affected by the solute and, consequently, the mixtures of both isomers with D2O show a lower consolute point. In the case of 4-methylpyridine the enthalpies are almost equal to one another, thus the phase properties of the mixtures with light and heavy water are similar. At higher methylpyridine concentrations the hydrophilic effect predominates; because of shortage of water, the hydration spheres around the non-polar parts of the methylpyridine molecules, built of water molecules linked tetrahedrally to each other by hydrogen bonds, decay gradually. Because of the equal polarity of the H2O and D2O molecules, the enthalpies of mixing for those solutions are almost independent of the isotopic composition of water.