Molar enthalpies of solution in water for thiourea (TU) and 1,3-dimethylthiourea (1,3-DMTU) were measured calorimetrically in the temperature range from 278.15 to 313.13 K at ambient pressure. Experimental results were used to derive limiting or standard (at infinite dilution) molar enthalpies of solution and hydration, changes in heat capacity due to the solution process and solute limiting molar heat capacities. Molar enthalpies for dilution of aqueous TU and 1,3-DMTU were also measured at T=298.15 K. The enthalpic coefficients for pair-wise interactions between hydrated solute molecules were estimated using the McMillan-Mayer formalism. The latter quantity was found to be negative for both TU and 1,3-DMTU, decreasing in magnitude on going from the unsubstituted solute to its N,N'-methylated analog. Hydration behavior of both TU and 1,3-DMTU was discussed in comparison with that of urea and the corresponding dimethylsubstituted derivative. (C) 2013 Elsevier B.V. All rights reserved.