The enthalpies of solution of 1,3-dimethyl-2-imidazolidinone or N,N'-dimethylethyleneurea in H2O and D2O were measured at 288.15, 298.15, 308.15, and 318.15 K. Standard enthalpies, heat capacities and entropies of solution (hydration), along with D2O-H2O solvent isotope effects on the quantities in question, were computed. The enthalpies of the solute dissolution as well as corresponding solvent isotope effects were found to be negative and decreasing in magnitude with increasing temperature. It was concluded that, in spite of forming the "heterocomponent" H(D)-bonds in aqueous solutions of the cyclic methyl-substituted urea derivative studied, hydrophobic hydration is the predominant type of the solute hydration and it is enhanced in D2O. (c) 2008 Elsevier B.V. All rights reserved.