The enthalpies of solution of 1,1,3,3-tetramethylurea in H2O and D2O were measured at 278.15, 288.15, 298.15, 308.15, and 318.15 K. Standard enthalpies and heat capacities of solution (hydration), along with D2O-H2O solvent isotope effects on the quantities studied, were computed. The enthalpies of the solute dissolution as well as corresponding solvent isotope effects were found to be negative in sign and decreasing in magnitude with increasing temperature. It was confirmed that hydrophobic hydration is the predominant type of the solute hydration and it is enhanced in D2O and weakened on going from 1,1,3,3-tetramethylurea to its cyclic derivative, 1,3-dimethy1-2-imidazolidinone. (C) 2009 Published by Elsevier B.V.