Molar enthalpies of solution in water, Delta H-sol(m), of thiourea, methylthiourea, ethylthiourea, dimethyl-1,3-thiourea, diethyl-1,3-thiourea, and tetramethyl- 1,1,3,3-thiourea were measured at T = (296.84, 302.45, and 306.89) K by isothermal calorimetry. Experimental results were used to derive molar enthalpies of solvation at infinite dilution (i.d.), Delta H-solv(m)infinity, changes in heat capacity due to the solution process, Delta C-sol(p,m)infinity and partial molar heat capacities at i.d., C-p,2(infinity),, at T = 298.15 K. The methylene group contributions to the enthalpy of solvation and partial molar heat capacity were -4.5 kJ.mol(-1) and 88.5 J.K-1.mol(-1) respectively, in good agreement with our earlier data for N-alkylureas and the literature data for various homologous series of alkyl-substituted compounds. The contributions of the functional groups to both Delta solvH(m)(infinity) and C-p,2(infinity) were derived by employing additive schemes and compared with our earlier data for urea and corresponding N-allcylureas. Hydration behavior at the i.d. of thiourea, urea, and corresponding N-alkyl derivatives were compared and discussed.