The enthalpies for dilution of solutions of 1,1,3,3-tetramethylurea (TMU), 1,3-dimethylethyleneurea (DMEU), 1,3-dimethylpropyleneurea (DMPU) and 2,4,6,8-tetramethyl-bis-urea (TMbU) in ordinary (H2O) and heavy (D2O) water were measured calorimetrically at 298.15 K. The enthalpy coefficients for pair (h(22)) and triplet (h(222)) interactions between hydrated solute molecules were estimated using the McMillan-Mayer formalism. The quantities of h(22) as well as the corresponding solvent isotope effects were found to be positive for aqueous TMU, DMPU and DMEU and negative for aqueous TMbU. The correlation between h(22) and standard enthalpies of solution of these compounds in H2O and D2O was established at the temperature in question. (C) 2011 Elsevier B.V. All rights reserved.