The enthalpies of solution of 1,1,3,3-tetramethyl-2-thiourea (TMTU) in ordinary (H2O) and heavy (D2O) water were measured at (278.15, 283.15, 288.15, 298.15, and 313.15) K and atmospheric pressure. Standard molar enthalpies and heat capacities of solution and hydration, together with D2O-H2O solvent isotope effects (IEs) on these quantities, were calculated. It was established that, unlike the process of forming aqueous 1,1,3,3-tetramethyl-2-urea (TMU), the dissolution of TMTU in both H2O and D2O is an endothermic effect over the whole temperature range studied, and the standard enthalpy-isotopic effect undergoes a negative-to-positive sign inversion nearby of T=304 K. Going from TMTU to TMU, the standard heat capacity of solution (hydration) and corresponding IE become less positive. (C) 2014 Elsevier B.V. All rights reserved.